Sounds are also contagious, whether they are emitted by a living thing or a simulacrum. The sound of someone breathing heavily in a movie can affect your own breathing rate. A commercial jingle is successful if it lands in your head and stays there, replaying itself unceasingly. You may not want a c.o.ke or a mattress or a pizza right this second, but the suggestion has wormed its way into your subconscious in case you grow thirsty or sleepy or hungry. And the contagion of sounds has a biological component: if a predator growled at our ancestor's ancestors tens of thousands of years ago, his insides likely vibrated to this growl, and he hightailed it away from the scene. Today, if my son, recently an infant, cries, I feel it viscerally. I can locate no anatomical mother-b.u.t.ton he is pushing, but I react as though there were one. It is as if he cries at a frequency that makes my interiors rumble-and I hightail it toward him.8 I can tolerate the sounds of my son's cries, but the sounds of other people's cries, as well as most sounds shared in a city, have made generations of urban dwellers first cringe, then fume, and then, finally, form Quiet Leagues, Anti-Noise Campaigns, and Noise Abatement Commissions. As early as 500 BC there were complaints about the noises of animals working (elephants trumpeting, horses whinnying) and men playing (gongs, drums, or just making merry). By the seventeenth century in London, the complaints began to find their organized center. These afflicted urbanites were subjected to not just babies but also street criers hawking their baskets, beans, bells, cabbage, eggs, or flowers to anyone within earshot. Chimney sweeps, chair menders, and tinkers hollered notice of their services; dogs yelped, roosters awakened, and street musicians added musical insult to auditory injury. Parliamentary action was taken against the musicians and their "devious and hurtful" sounds. By the turn of the twentieth century, New York City had joined the din against the din. The din itself had changed: no longer was the urban soundscape full of noisome animal sounds; machines had overtaken them. The polemics against noise cited the incredible cacophony of engines revving, honkers honking, pneumatic drills, pile drivers, and wheezing trucks. This was on top of the people playing piano poorly inside and saxophone poorly outside. All the singing, crying, rattling, whistling, thudding, slamming, ringing, rasping, and alarming was bad for health and for habitation.

By the time Lehrer and I finally left our position on the corner, my ears were well nourished, stuffed full. I had almost stopped listening to the city, and that may be why, when Lehrer was saying, ". . . like that siren we heard over there," I was struck. I had heard no siren. How I could have missed one of the noisiest features of the urban soundscape is beyond me. What Lehrer was describing partially explained it, though: the way the city sounds simply is not the sum total of the sounds in it: "You know, we can't really record that [siren sound] because if you recorded the siren so far away all this other, closer noise happens," crowding the sonic scene. "If you record the siren closer, you get a clearly defined sound. But then if you just take that and put it into your soundscape when you're making an environment, it sounds wrong."

In the real world, the sound is reverberating in a particular way based on the structures it is pa.s.sing; the sound arriving at listening ears is changed depending on what lies between the ears and the siren. The pitch and loudness may seem steady, but they are changeable, and they are different if the listener is a block or three away. The Doppler effect will be different based on not just the speed of the ambulance bearing the siren, but on the direction of your movement relative to that ambulance. Viewed this way, every moment of listening in a city is unique, a sonic landscape painted for the moment and then washed away.

Even temperature changes our perception of sound. It is not our ears that are changed, nor the sounds themselves (for the most part), but that different temperatures control how far and where sounds will travel. Perhaps you have a memory, held in your body as much as in your head, of being outside in the wilderness, in a wide open area, the sun beating down on you-and experiencing an intense silence. Or, relatedly, on a clear night outside, hearing distinctly what is going on in a tent three campsites away. Lehrer's siren carriers farther on the city's coldest days, when fingers are balled up in mittens and footsteps clop loudly on sidewalks.

We can turn to the sound-making habits of animals to explain what is happening here. Natural selection naturally selects the animals who send signals to their potential mates that can be most clearly received: so, in many cases, evolution favors those who intuitively know how to best send a sound signal through a medium-say, air or water. Not all air (or water) is alike: often it forms a kind of layer cake, in which each layer is at a different temperature or a different pressure. For instance, as you dive deeper into the ocean, the pressure steadily increases: the greater depths have a much higher pressure than the shallower waters. On land, each night the earth cools, and in the morning the radiated heat makes the ground cooler than the sky: here, the lowest layers are cooler than the higher layers. We can think of the layers as having certain "sound speeds"-speeds at which sound can travel.9 It turns out that sound travels more slowly in warmer air (or lower pressure) and faster in cooler air (or higher pressure). If the sound is traveling along a cool layer and there is a warm layer above it, the sound will spread into it and diffuse. On the other hand, if the sound is moving through a warm layer and there is a cool layer nearby, it will continue to travel along that warm layer, which now channels the sound farther before it weakens and fades away.

This is why you will hear the most birds singing at dusk and dawn. After a cold night, when the earth is chilled, the ground layer is cool and the layers above the treetops are warmer: a temperature inversion from the ordinary arrangement of the ground feeling warmer than the air. A bird singing at dawn can send his tuneful song traveling much farther along the treetops than it otherwise would. This is good news for singing birds, who are hoping to reach as many other bird ears, especially of the female variety, as possible. Likewise, few birds sit around on the ground calling to one another in the middle of a sunny day, and temperature is again the cause. In a warm layer of sound, their calls get scattered every which way. The message they are sending to the bird a skip and a jump away may not even reach them, the sound disappearing into the ether.

Similarly, there is a fine whale radio channel in the ocean-in a layer of ocean. The layer sits at about a thousand meters below sea level in the North Atlantic, where the pressure is not yet too great and the temperature is not too cool. This allows for long-distance sending of sound signals, on the order of miles, channeling the sound horizontally to distant whales' waiting ears. Some twenty hertz sounds made by fin whales, who live in deep water often under ice fields, are speculated to travel for hundreds of miles. Though these whales are highly social, they are also often quite well dispersed, and make these low-frequency sounds to keep in contact with one another. Their calls could travel even farther but for interference from other sounds made by other whales, the ice itself, or, increasingly, by human beings-from ship traffic, undersea explosions, and Navy sonar.

If sounds travel differently along temperature channels, the seasons of the year could also be considered to be separate channels changing sound perception. After a big snowstorm, the city is noticeably quiet, the snow s.n.a.t.c.hing the din and burying it under its chilly cloak. Packets of snow occasionally flop noiselessly from lamppost heads to the ground below. Few cars roam the street, except the snowplows grittily sc.r.a.ping the asphalt under their wheels. Boots squeak as they carry the weight of bodies over a sidewalk of snow.

My footsteps, quiet in sneakers, were reflecting the spring. We descended the stairs at a subway entrance-the very subway that had rumbled below us earlier. Before Lehrer and I said our good-byes, we lingered in the anteroom of the subway, struck by the sheer number of noises in the s.p.a.ce: the clunk of the turnstiles, the snippets of quickly pa.s.sing conversation, broadcast announcements accompanied by impressive static. When a train actually arrived, it brought its own reverberant broadband rumble, the squeak of its brakes, a swarm of noisemaking commuters, and the whirr of its accelerator. Those entering the station yielded to those leaving, and then the tide turned again. Turnstiles dinged at a prodigious rate, recording the paying fares.

"No one bothered to make that consonant at all," I reflected, listening to the different turnstile tones overlapping.

"Minor second," Lehrer responded in a flash, describing what we were hearing. He whistled it. "People don't like the minor second."

There is a solid scientific basis for this aversion. The pitch of a sound is its most audible frequency of vibration, but that particular vibration is just one of many produced at the same time with any sound. A note played on a piano, for instance, may be heard as one note, but it "includes" many other pitches that help make up the sound we hear. An experienced musician may be aware of these pitches; the rest of us are likely not. The series of pitches hidden within one note are called overtones, and they correspond to other notes on the piano. Hit a middle-C and it vibrates at 262 hertz most loudly. But the note at 524 hertz, the C an octave up, rings out as well. This hidden vibration is the first overtone. The fifth, a G, is next, followed by the fourth, the major third, the minor third, and so on. You might not hear these overtones, but you are surely aware of them: an octave sounds pleasing to our ears, as does a fifth or a third. When one gets to the outer borough of the overtone sequence, though, the sounds are more dissonant. The minor second is well in the outer boroughs. Similarly, the tritone, an augmented fourth or diminished fifth, is so dissonant and unnerving that it was thought to be the work of darker powers, and it came to be called diabolus in musica, the "devil's interval." In the Middle Ages it was prohibited in music.

I wondered if this minor second was having any long-term psychological effect on the transit workers stuck in the underground booths.

One of the old uses of the word silence netted and pinned forever to the page in the Oxford English Dictionary is the nineteenth-century's "a want of flavour in distilled spirit." I thought of this that night as I sat down to listen to the audio recording of my walk with Lehrer. Back in the silence of my office at home . . . wait. There was not silence but relative silence. I knew that a few blocks away, a highway hummed. There were regular sounds of apartment living coming out of the back windows my office overlooks. Clothes languidly swished in a warm-water cycle nearby. And the sounds of the day left a ringing in my ears, representing all the sounds we hadn't heard outside. Our brains make sound out of silence. Noise seems to be the flavor we are designed for.

As I transcribed the recording I made of our conversation on the walk, I was struck by all the noise the recorder, with its high fidelity and indifferent attention, picked up that I had missed at the time. It captured and preserved its own rhythmic banging against my leg as I strode. It noticed my sniffing, an indication of the chill I was collecting as the walk went on. I was surprised to hear on the tape how often the wind rode in to wash out any other sounds. Once, the curls of a laugh rose above it; other times, it erased everything else. I listened for any extraneous sounds of Lehrer: snapping snaps, sighs, a whistle while he inhaled. He was silent. At the end of the tape, we said our good-byes, and the sounds of the city swallowed him up.

Taking a walk entirely for the purpose of listening, I had still missed many sounds. But something else had happened. What I heard had morphed from noxious urban noise into being the characteristic, flavorful clatter of my city. I enjoyed the roar of traffic and the buzz of flies; I looked at pigeons hoping they would coo; I stared down pa.s.sersby, silently egging them on to hum or cough. I counted squeals and squeaks and squawks and measured them against whines and whistles. Each sound felt invited, a pleasure. Welcome, sound.

1 Hertz (Hz) is the standard measure to describe the pitch at which we hear a sound. Sound is simply a wave of pressure moving through s.p.a.ce; the number of hertz indicates the number of those waves per second. The higher the hertz, the higher the sound we hear.

2 And what of giraffes? One might get the idea from children's literature that giraffes are silent. Not only do they whimper and grunt, but giraffes also emit an infrasonic, low-frequency sound when they "neck stretch," reaching the neck back over the body, and when tossing their heads up and down.

3 Alas, sometimes one hears less, too: not only does hearing loss occur naturally with age (by this writing I have lost ability to hear the top 6,000 or so hertz of my original hearing range), but it occurs with exposure to any sound at all-even at nonSpinal Tap levels.

4 Named for Alexander Graham Bell (his second l lost to history) for his role in sending sounds across telephonic wires to waiting ears.

5 Among the many reasons to appreciate bats, this one stands out: bats are the primary responsible parties for our not being eaten alive every day by mosquitos.

6 I have read, but not heard, that their pulsing rate quadruples with each degree centigrade the mercury rises.

7 Ultraviolet light is visible to many other animals. Plenty of bees and birds, for example, use the reflection of ultraviolet light to find food (reflected off a bull's-eye around the stamen in the center of a flower) or a good mate (whose feathers may reflect more UV light if the animal is healthy).

8 Indeed, some research using fMRIs found that two areas-the amygdala and the ventral prefrontal cortex-in the brains of parents grow more active at the sound of an infant crying. Not so nonparents, whose brains showed a bigger response to laughter than to crying.

9Sound is always the same kind of stuff: compression waves propagating through a medium, causing particles to hand off the vibration to their neighbors. We hear these waves as pitched higher or lower, or as being more or less intense-but they can also travel faster or slower. Counterintuitively, while sound moves at around 1,100 feet per second in the air, it races along at somewhere on the order of 5,000 feet per second in water. Though water is denser than air, which slows down the pa.s.sing of the waves through the water molecules, it is also stiffer than air, which speeds up the sound. Sound pa.s.ses through solids, which are stiffer still, at fifteen times the rate as through water.

"The only true voyage . . . would be not to visit strange lands.

but to possess other eyes, to see the universe through the eyes of another, of a hundred others, to see the hundred universes that each of them sees, that each of them is."

(Marcel Proust).

A Dog's-Nose View.

"A beagle pulling on a long leash trotted by and unceremoniously defined the corner of the trash pile by urinating on it."

This entire project sprang from a walk with a dog; it seemed apt that it end with one. I spent sixteen years walking with (and informally studying) Pumpernickel, a curly haired, sage mixed breed. Through her choices, the subjects of her attention, what she balked at or lunged toward, I began to see her world. By minding her attention, seeing what she saw from two feet off the ground, and observing how much she seemed to smell her way through the world, my own perception was changed. I began to see how horrible a long block with no trees or lampposts was: where could one receive word, through the markings of other dogs, who has been around? Where could one leave word oneself? Despite my never once attempting to communicate with others by peeing on the street, I picked up Pump's aesthetic preference for streets with a lively set of street furniture, trees, and other curbside paraphernalia.

During her life, we developed a wide variety of walks together geared to what I imagined her view of the world to be. There were into-the-wind walks, during which she kept her eyes closed into slits and her nose in the air, nostrils working mightily. We took smell walks: instead of racing to take a walk defined by me for its length or its destination, we loitered at every place she wanted to smell, as she inhaled her view with her nose. As she grew older, we took walks that were largely episodes of sitting, in a field with ample olfactory vistas and plenty of dogs upwind. Most dog walks are done to allow the animal to pee or to get exercise. While those are sound reasons for a walk, what about walks to see the world? To interact with other dogs? What about walks to smell new smells?

Because humans are not smell-centered, we have difficulty imagining how rich in odors the world is. That is a constraint of our eyes: the picture they paint is so vivid that we a.s.sume there is no other way to make sense of the world but as a series of things to see. For most other terrestrial mammals, though, on four legs and with their noses near the ground, the world is perceived through odors. Kenneth Grahame, in The Wind in the Willows, introduces the genial Mole to school us: "We others," he writes, "who have long lost the more subtle of the physical senses, have not even proper terms to express an animal's inter-communications with his surroundings . . . and have only the word 'smell,' for instance, to include the whole range of delicate thrills which murmur in the nose of the animal night and day, summoning, warning, inciting, repelling . . . those caressing appeals, those soft touches wafted through the air."

Mole must lead us stump-nosed, blunt-seeing humans to imagine what it might be like to "see" in this way through metaphors: metaphors from sound (murmur), tactile sense (appeals, touches), and evocations of emotion (thrills, inciting, warning). We can come up with a vivid description of how a place we visited looked; but how it smelled? We are left with vague comparisons ("like a summer's day") that are evocative but not specific; names that tell us nothing about the quality of the scent (the smell of garlic or fresh bread); or superficial quality words (foul, lovely, delicious, spicy). What smells are very good at is beckoning memories forth: a whiff of pipe tobacco reminds me of the smell of the inside of my father's desk when I was a child and he was still a smoker, of the sound of his footfalls and the jangle of the change in his pants pockets, of what it felt like to have him listen to me and smile. The smell, like the memory, is entirely personal. It cannot be shared with the ease that an image, rendered in ink or oils, can be experienced by hundreds or millions of viewers.

By now, it is well known that dogs are "good" at smelling. As we humans open our eyes and see the world, dogs come out of sleep with both nostrils working. A dog's nose is remarkably well made for this task. The inside of the nose is a labyrinth of tunnels lined with specialized olfactory receptors waiting for an odorant molecule-a smell-to land on them. In the back of the nose is an "olfactory recess" separated from the main respiratory pathway by a bony plate, allowing smelling to be distinct from breathing, and letting odors loiter for a long time to be considered. Though we tend to think that only some things are smelly-a spring bloom, a trash can, a new car, a bus's exhaust-just about everything has a scent. Anything with molecules that can be "volatile," that can evaporate into the air and travel toward a receptor in someone's nose, smells.

The dog nose has hundreds of millions of receptors in that nose; they even have a second kind of nose above the hard palate of their mouth, called a vomeronasal or Jacobson's organ. Molecules such as hormones that do not stir the receptors of the nose to fire may find a rousing welcome here. All animals house hormones, which are involved in bodily and brain activities, and those hormones we emit, called pheromones, are detected by the vomeronasal organ. This is how a dog could detect another dog's stress or s.e.xual readiness in a spray of her urine left on the ground.

Dogs are called macrosomatic, or keen-scented, while humans are called microsomatic, or feeble-scented. It is not for lack of equipment, though: almost 2 percent of our genome, the entire blueprint for making a human being, is dedicated to coding for olfactory receptors. Think of it! One out of every fifty genes is committed to making cells that can detect smells. So smells are important to us. Without smells in our lives, we become desperately unhappy-foods are not enjoyable, the environment is flattened-while some smells bother us excessively.1 But for much of the day, we go about our business un-smellily: we do not smell overmuch. For humans, odors tend to be either enticing or repugnant, alluring or foul, evocative or evaded. To a dog, the world is terrifically smelly-but not in the way we think of smelliness. To the dog, smells are simply information. Their world has a topography wrought of odors; the landscape is brightly colored with aromas.

Biologically, the human nose works in the same way as the dog's. Odors are swept into the far reaches of the nose and land in receptors-a few million of them. But that is hundreds of millions fewer than the dog bears.2 The difference in number of cells translates into a difference in kind. Dogs detect odors at one or two parts per trillion, unimaginably more sensitive than we are. One part mustard, one trillion parts hot dog: dogs can detect the mustard.

To begin to understand what a city block really smells like, there seemed to be one clear course of action: ask a dog. So it happened that one day I set to taking a walk around the block for this book with Finnegan, the earnest, playful black dog in our home now. I began by asking Finnegan about his interest in accompanying me, in showing me the odors of our block. From the way he was plopped languidly on the sofa, head relaxing off its edge, he did not look inclined. But on my second request, he leapt up, consented to having his leash snapped on, and trotted out our door alongside me.

Finn pushed out into the fresh air with enthusiasm. I followed him. Then we . . . stopped. It had occurred to me to ask his preference in our walking route, so instead of pulling him left (parkward) or right (cityward) when we exited the building, I stopped on the steps. Finn, ever cooperative, stopped as well. He perched on the top step, projecting his snout proudly ahead of him. As a steady, light stream of people walked by, they pushed air out of the way as they pa.s.sed, occasioning plenty of sniffing by Finn. If someone so much as turned in his direction, he ducked his head and let his tail wiggle his body with antic.i.p.ation and excitement.

I waited for him to make a move. Between pa.s.sersby, his body was still, only his head reacting to the activity on the street. The day was especially windy, and a sad old flag on a building across the way whipped around its pole. Finn perked his ears at the snap of the cloth, the bang of the flag rope against the pole. A sound Scott Lehrer could use for a shot of a seaside New England cottage hit by an ocean breeze, I thought. Our urban wind carried the sounds of someone hollering down the block, and tumble-weeded a plastic bag, belly full of air, along the tops of the parked cars.

We stood on the steps for several minutes. At last, I realized that should I not start us moving, we would not be having a walk at all, but a rousing "stand." Were Finn any other kind of animal, he might have bolted as soon as we got outside. So I chose a direction and began walking. As we set off, I watched Finn's attention. My audio recorder was again useless here, as it had been with my son. Instead, I would have to let Finn tell me what he saw by observing where he went, where he loitered, how his ears bent into focus, and how his tail measured his mood.

Straightaway, he sneezed, then licked his nose. I had to think that this was not significant, a reflexive reaction to a pollen tickling him. Later, I would rethink this. Finn pranced along by my side. He held his head high, and it gently bounced up and down with his stride. His gaze lightly touched the wall to our left, a garage door closing, a dog pa.s.sing on our right. A driver honked a loud honk; Finn did not pause, but his eyes narrowed and his ears flattened against his head. Pressing down his ears should make the sound quieter, if not absent-the canid equivalent of our covering our immobile ears with our hands.

It was not long before he stopped outright. He licked his nose again. A squat, unimpressive bal.u.s.ter caught his interest. It appeared to be made of poured concrete and held up an iron railing: a short skirt worn by the building to protect it from us. Finn peered at it, nose millimeters away from the surface. He even touched the surface, moistening it with small nose prints. I tried to follow his nose-gaze. I could see a palimpsest of messy splashes of drying liquid, some more messy or more dry than others.

This was, of course, the motherlode for dogs: other dogs' urine. Visible generations of urine splotches lay one (roughly) atop the other. After some satisfying-looking sniffing, he moved to the next urine post. And the next. I noticed with some chagrin that there appeared to be a half dozen bal.u.s.ters ahead of us before we reached the corner. Given his renewed attention at each, I got the distinct impression that to him they were not at all alike. Each told tales of different visitors, some well fed, some ready for s.e.x, some aggressive, some ill. Maybe some sad, longing, suspicious, or delighted.

I began to imagine the sidewalk as a place that conceivably held traces of other people's pa.s.sage-of their mood and health and habit. In what form I might see those traces was another question. Other people's trash was the first thing that came to mind, unhappily: in a city, there is always evidence of others' pa.s.sage by the things they let drop through their fingers. But there are many more subtle cues. The fog of warm air that hits me as I pa.s.s a parked car tells me that someone quite recently turned off the ignition and left. The disruption of fallen petals or leaves on the sidewalk works like reverse tea leaves to tell the story of how many people have swept by. Look up when you see a flurry of cigarette b.u.t.ts on the ground and you are bound to see the entrance to a commercial building. (And it is likely after lunch break.) Some years ago I began noticing, then collecting, stray single gloves or mittens lying forlornly on the ground, displaced from the hands they had been warming. These melancholy creatures, always frozen in an awkward or pleading pose, indicated recent pa.s.sage of someone busily doing something requiring a free hand. I found more right gloves than left, probably a reflection of the overwhelming right-handedness of people, and the inclination to remove a right glove to do something requiring dexterity: take out one's wallet, punch in a phone number, retie a shoe.

I did not know more than this about the people who had pa.s.sed, shedding their gloves. Certainly, they now had chilly hands. But were they happy or sad, old or young, healthy or ill? Did they live nearby, were they pa.s.sing through, did they walk with someone or alone? Apart from the difference between a child's mitten and an adult's leather glove, any personal details eluded me. I did not yet have the perceptual ability to determine who those recent pa.s.sersby were-though I suspected that information was there to be discovered. As Finn would soon tell me, the dog can see it.

One thing puzzled me about Finn's bal.u.s.ter examination. Once I saw what he was sniffing, the graffitied sprays and splashes were easy to spot. But Finn did not always go directly to the message. He sniffed around it, as though not sure where it was. A dog's vision, though not nearly identical to ours, surely lets him see what I saw here. Finn could see the urine mark, but he was "scanning" the area, just as we scan a scene with our eyes. To see a scene is not to stare fixedly at one point; it is to open our eyes to everything in front of us, looking to and fro. Similarly, to smell a scene, Finn approached it from the side, from above, sniffing the air to see if the artist who concocted this particular odor splotch was anywhere nearby. A dog can smell something different in each noseful-and there is something different there to smell. This taught me something about smells: they are not at fixed points, nor are they static and unchanging. They are a haze, a cloud, spreading out from their source. Viewed as odors, the street is a mishmash of overlapping object ident.i.ties, each crowding into the next's odorous s.p.a.ce.

I leaned in closely to watch Finn's sniffing. He stopped at once, of course, possibly wondering why I was giving him the eye. What I did see was that he was sniffing fast: dogs can sniff up to seven times a second. Humans sniff about once every two seconds. If you try hard, you can do a good run of a dozen sniffs, but then you must stop to breathe, and in exhaling, all that good scented air gets expelled. Should you want to avoid smelling something, give a good strong exhale from your nose. If you are out of its cloud when you next inhale, your olfactory receptors will be none the wiser.

Not only do each of Finn's nostrils pull in odors, they collect a sampling of different slices of the world, which may allow dogs a kind of stereo olfaction. Just as we humans locate the source of a sound by unconsciously and instantly calculating the difference in its loudness. .h.i.tting our left and right ears, a dog can gauge the difference in a smell's strength between his left and right nostrils. And I was happy to learn that by watching dogs' nostrils very, very carefully, researchers have found that dogs use their right nostril first when smelling a new but nonaversive scent (food, people), and then switch to the left nostril once it becomes familiar. By contrast, both adrenaline and the sweat of veterinarians (yes, the researchers collected vet sweat) prompted a bias toward sniffing with the right nostril. These nostrils, and the olfactory cells they lead to, send information to the same sides of the brain: right nostril, right hemisphere. The researchers conclude that right-nostril activity is a.s.sociated with stimulating an arousal response, of aggression, fear, or other strong emotion. The left nostril, like the left hemisphere, is involved in calming experiences. In theory, if you looked closely, you could see if a dog considered you friendly or not by watching which nostril is sniffing.

All this sniff rumination got me back to Finn's sneeze. Starting our walk, he sneezed a good sneeze. After I had interrupted his sniffing, we resumed walking, and he sneezed again. He was cleansing his palate. Sneezing is a dog's way of clearing everything out of the nose, so the next good stench can be inhaled. It is the dog's version of the polite, throat-clearing cough timed to end one conversation and move to another, or the self-conscious ahem done to break the silence in an elevator or other closed social s.p.a.ce.

Similarly, by licking his nose, he was readying it to catch things to be smelled. You may have noticed that the world outside your door smells brightly new after a rain, when the ground is wet-and especially so in summer, when every surface is well baked and warm to the touch. Those molecules of odor that have settled on the ground, latent odor traces, are unlikely to be detected by a pa.s.sing nose. But warm air-warmed by the sun or from the inside of a dog nose-makes odors volatile and more likely to be sniffed. And wet air (or noses) allow for better absorption of an odor. This is also why Finn may lick or nose-print a surface: he is not trying to eat it; he is attempting to get the odor closer to the vomeronasal organ, where it can be smelled further.

Although we had resumed walking, it was at a slow pace. We made regular stops for Finn to sniff the ground, an errant hubcap, or a paper bag on the sidewalk, or to stand and sniff into the wind. What olfactory fireworks must reach his nose on a brisk wind! Smells not just from our street, but from around the corner, from down the hill. Scents from New Jersey! Scents from great heights, from the past, from the place we were walking toward.

At this pace, I began noticing things on our route I had never noticed before: small faucets attached to sides of buildings; bra.s.s pegs in the sidewalk that are, I subsequently learned, part of a national registry of such pegs;3 the difference in the ratio of shade to sun on the north and south sides of the street. After stopping at each block's fireplug, I realized some had sentinels: short guard posts flanking the fireplug. I suspected these represent a historical glimpse of unhappy automobile/fireplug interactions. I began looking a little too closely at the innumerable spots on the sidewalk (look! they're there!): black bruises that on examination are darkened, smushed gum or splattered liquid.4 At a tall apartment building, something caught Finn's attention at the entrance, three short steps above street level. I followed his gaze. With much fuss, two people were settling a large, older gentleman standing in the doorway into a wheelchair. It looked difficult and exhausting for all involved. Seeing us looking, the man gaped back at us. I smiled and, being a human being, turned away out of politeness. Surely this man did not want me staring at his difficulty in sitting down. But Finn was transfixed. Not only did he continue to stare, he locked his legs, settling his weight on his heels when I encouraged him onward. As the leash became taut between us, I looked back at Finn. He was certainly being impolite.

But, of course, no dog is polite or impolite. It is we who attribute these characteristics to them. Dogs are perfectly culturally ignorant. Despite their neat insinuation into our homes, they do not notice or concern themselves with our customs. Perhaps a dog may learn to "not stare" if an owner punishes him each time he does, but not necessarily: the dog may just learn watch out for your owner, she's coming for you. Politeness is a human concept, and it is at best bizarre to imagine that dogs have it.

On the other hand, I could feel, and I suspect the wheelchaired man could feel, the fixedness of Finn's gaze as discomfiting. I knew Finn was just looking, but I felt I needed to relieve the man of being watched, even if only by a dog.5 It is a testament to the power of the dog's gaze that we can be unnerved by it, as though the dog were seeing us for who we really are. Both Finn and my son, in their ill-mannered staring, have done more than any psychologist or sociologist to make me more aware of how material gaze is.

When I convinced Finn to move on, I thought to pay attention to where other dogs we encountered on the street were gazing. Nearly all were more interested in getting their noses into Finn's nether regions or wagging face-to-face than in staring at me. Often, though, a dog looked at me-or maybe smelled me. It felt like an invitation. The dog's gaze is simply human.

One dog, an energetic brown-and-white houndy type, galloping toward us against the restraint of the owner he was pulling along, seemed to smell particularly interesting to Finn-and Finn to him. They wagged mightily at each other, tails high in the air, the wags taut and vigorous, then set to an intensive sniff dance. I call it a dance because they moved together, like long-term dance partners, doing some behaviors at once, and others in response to each other: first, mutual sniffing of the low bellies; then back upright, faces close; then a circle step, caused by each trying to get his nose right at the base of the other's tail. I smiled up at the person at the end of the dog's leash, which seemed appropriate as our dogs were getting familiar. She was tall, wearing a raincoat on an unrainy day. Her hair looked expensively cut, but her face was weary, harried. She barely managed a smile back. I thought about what information Finn was getting about her dog with his sniffs: mood, opinions, disease? My Philadelphia physician Bennett Lorber sniffs the tissue samples of his patients. We humans, trained sufficiently, can detect the scent of illness. When physical exams were more literally physical and less mediated by machines, it was common for trained doctors to identify disease in their patients by smell. What were charmingly called "lunatics" in the nineteenth century were said to smell "like yellow deer or mice" (that presupposes that one knows what those animals smell like; perhaps once you get the chance to smell a lunatic, it becomes obvious). Typhoid smells like freshly baked brown bread; the measles, of "fresh-picked feathers"; scabies is moldy; gout, like whey; tuberculosis, reminiscent of stale beer; diabetics, unbearably sweet, like rotted fruit.

Abruptly, the dog bolted and the two of them, wafting their odors in their wake, headed off to the next entertainment. Finn sniffed after them, then turned and trotted away. I refrained from sniffing after the woman. For a few minutes, no one pa.s.sed us on the sidewalk. Finn took to conducting a detailed examination of the objects of the street. He poked his nose into all sorts of openings. Low bas.e.m.e.nt windows abound in my neighborhood, I learned through following him, and each seemed to have its own currents of dog-alluring scents. s.p.a.ces between cars were exciting, as they allowed sniffs of the filaments of odors from the other side of the street. We spent a lot of time with lampposts. I would characterize most of Finn's behavior as merrily interested, but as we approached a large Dumpster parked by the sidewalk, I got a taste of his alarm, too. A plastic tarp over the Dumpster had come loose from its moorings and slapped against itself. It was a distinctly different sound than the whip of the yellow "do not cross" tape surrounding a wet square of concrete nearby. Both seemed to spook Finn with their air-snapping and whipping movements in the wind. But he was brave. He held his ground, leaned as far forward as he could while keeping all four feet planted, and inched his nose ever closer to the mysterious tape. Ten seconds-seventy sniffs-later, he determined it was harmless, and even dull, and we moved on. I listened for the other sounds of wind on the street. I tried to listen with the ears of a sound engineer. The round-leaved trees rustled differently than large oak leaves-and both were distinct from the sough of small linden leaves. Finn did not seem to attend to these one whit.

He had something else in mind. Suddenly, after a long slow ramble, he was pulling me, his nose in urgent pursuit. We wound up at the stairs outside a short apartment building. Every week, I watch spellbound as his nose leads him on a wending course to locate the handful of well-s...o...b..red tennis b.a.l.l.s that have been waiting in a field of ivy since we visited the prior week, but this time his nose seemed to have led him astray. There was no ball. There was nothing. Only a still, closed door in our face.

Three minutes later, we were on the stairs to greet a woman I recognized from morning dog walks, leading her charge out for a bit of midday relief. She had, she explained, just returned home. I goggled a bit and tried to explain to her our presence on her stoop, while Finn happily wiggled. Shortly, he was ready to move on, and it was I who wanted to loiter and think about how he knew where she lived.

I should not have been at all surprised. Buildings do not typically wear prominent evidence of their tenants, but each person leaves tracks, individual odors in our footprints, even in the smells that radiate from us into the air we move through. Catch an elevator just after it has dropped off a perfumed rider or a smoker and you see what I mean: the evidence of her presence still hangs in the air. Whether trained in tracking or not, dogs can detect our individual odors, and even the direction in which we have gone. Each step we leave has, over time, a slightly different quant.i.ty of odor than the one before and after it. The newest step is the smelliest; the oldest is the least smelly. For tracking dogs, five steps is plenty to determine direction through this change of odor concentration.

Finn saw this woman's recent return through her steps. The city block, for him, was covered with evidence of familiar and unfamiliar people and dogs pa.s.sing by. Each time I step out my door, I see more or less the same block. No wonder Finn stops when we exit: it is a wholly new street, wearing odors of the six hours since we were last outside, waiting to be sniffed in.

A long stretch with nary a tree guard nor lamppost nor fireplug to visit, and we were moving along at a decent clip. Even while I aimed to observe what Finn observed, I found myself mostly observing Finn. He was, after all, a part of the city block, I reasoned-as were all the other dogs we routinely pa.s.s. I focused on his bearing. He held his tail high, a friendly sickle curved over his back. He was doing a very particular kind of walk. Dogs, like us, can launch into any of a variety of strides, from walk and pace, to trot and amble, and, in a pinch, gallop. Between bal.u.s.ters, Finn walked: his left rear foot gently chased his left front foot, which started his right rear foot chasing his right front foot. This walk is considered "sloppy" by trackers: the footprints walking dogs leave do not directly register one on top of another; instead, the right front and left rear slightly overlap, leaving a distinct mark for each pawprint. Watching Finn walk, his hips swaying slightly as he moved, I felt that "sloppy," technical or not, was the wrong word for this stride. Maybe languorous or unconcerned. It was an easy, carefree movement that likely took as much energy as my putting one foot in front of another.

In this long empty stretch, though, Finn downright trotted. In a trot, the rear legs catch up with the front; each stride finds a dog with the feet on one side touching toes while the feet on the other side are spread as far apart as possible. When I really run with Finn, he is forced to gallop, a stride during which, for a moment, no leg is on the ground. Finn's gallop has his rear legs pushing together to launch him forward. On one street, a dog approaching us was pacing: the legs on one side moved together, followed by the legs of the other side. It made him rock from side-to-side, a comic gait for a dog to wear. Indeed, walking with the pacing dog was a man with his hands in his pockets, the dog's leash slung around his wrist. Striding without swinging his arms, he seemed to "pace," too. Evan Johnson, watching gaits with me, observed that the gaits of mammals and reptiles are unalike: in mammals, the upper back and shoulders typically move in opposition to the pelvis; reptiles are back-and-forth, moving the legs on one side of the body at a time. This pair looked positively reptilian.

Finn and I approached the start of our walk, which was also its end. Recognizing the building, or its stairs, or the smell of the flanking bal.u.s.ters (I knew not which), he headed toward the door without hesitation. Again I paused, this time to see if Finn's gaze looked different to me at the end of a walk than at the beginning. I hardly got the chance. A small child tottered over, delight on her face and a bonnet on her head. Her mother had a hand on a stroller and another on a phone; her mind was elsewhere. Finn lifted his head and simultaneously turned his body to face the youngster. I heard the implied "Doggie!" as the girl reached for Finnegan's head. At this he ducked, moved away, and the mother suddenly reached over and s.n.a.t.c.hed her child back.

Finn looked up at me balefully. I commiserated. We like to touch dogs. But do dogs like to be touched? Yes and no. There has not been much research into the dog's tactile perception, but studies have been done with horses and humans. In both species, tactile sensitivity is not the same throughout our bodies. We know that our fingertips, lips, and genitals can be extremely sensitive while parts of our back and legs are relatively insensate to simple touch. (This, too, matches areas where we are more or less disturbed by accidental public bodily contact.) Horses have, it turns out, a high sensitivity on the sides of their bellies-right about where a rider's legs fall. They can react to pressures lighter than humans can detect. This might explain why some horses seem unresponsive to a rider: they may be getting too many tactile signals for any one to be meaningful. Dogs, too, probably have highly sensitive areas, and would prefer not to be touched everywhere, or pet hard. There is evidence that rhythmic, firm strokes are relaxing for horses and dogs-but each dog has his own tactile body map. Look at how a dog touches other dogs, and where he licks himself: that might indicate areas of special sensitivity. If one's whole body is involved in experiencing the world, the "personal s.p.a.ce" we feel must be mapped directly from it.

My walk with Finnegan had been an ordinary walk: very little of moment had happened. Nothing one would bother to report to someone waiting for you at home. But, watching Finn, what counted as momentous changed: I saw how our world was colored with scent. Our block was a patchwork of smells, and the story of its day was readable in it. Through minding his attention, I remembered the smell of childhood, ripe as the smell of crayons, the must of an old book, the smell of a new car. I saw the array of bal.u.s.ters, windows, and detritus at dog height. I noticed myself in the scene-and that I had been blindly stumbling through my walk to work, my walk to the store, my walk to the car, missing the details that Finn detected.

Finn and I stepped inside and settled down together on the couch. I closed my eyes and smelled his fur: the top of his head smelled like fast-moving rivers; the curl of his ears evoked a warmed window seat. I could feel him sniffing me back.

1 Smelling through the nose is "orthonasal olfaction." There is also "retronasal olfaction": from the mouth, into the nose from the back. That is why if you plug your nose when you eat, it is hard to identify what the food is that you are lolling around on your tongue.

2 How receptors and the brain code the odors as distinctive smells is a live question for science. There is not one receptor for a single odor, nor is any one receptor activated by just one odor. There is no single receptor for roses, nor for coffee, nor for the smell of a baby's head. An odorant molecule fits in many receptors; and many receptors fire to many odors.

3 These "geodetic survey marks" are all over the city. The first were placed in New York in the early nineteenth century as part of a land survey. Each one's coordinates-alt.i.tude, longitude, and lat.i.tude, and distance from other marks-is noted, and one can look to see if these numbers have changed over time as buildings settle or waters rise. The marks are set in stable places, where they can be found much later-on public buildings, monuments, in bedrock-and have been made of steel, bra.s.s, rock, or other material resistant to decay or rust. Though the survey marks still have their original use, there are also entirely non-official geodetic treasure hunts on which visitors collect sightings.

4 As a long-time city walker, I have done some examining of these spots. While the prevailing view is that they are all spent gum, covered with city grime, the teardrop-shape of some suggest a droplet-as of a sticky liquid or a phlegmy sputum. When I began seeing the spots on the asphalt where buses stop for pa.s.sengers, I realized that they were caused by some viscous liquid or oil that the engine perspired while it rested.

5 To be sure, he was likely just smelling, but the eyes follow the direction the nose points, just as our nose sticks out at the person at whom we look (but we are not "nosing" the poor fellow).

"You know my method.

It is founded upon the observation of trifles."

(Sherlock Holmes).

Seeing It.

"As it turns out, I was missing pretty much everything."

I plopped on the floor and rummaged under the bench by our front door, feeling for my shoes. I needed walking shoes. Today I would return, alone, to my first route around the block, walk it again, and see what I saw.

Since the time I took that first walk, the city was substantively unchanged. Sure, some buildings were erected, a few were felled; many trees were lost in storms, a handful were planted. There were more bicyclists on the streets and more people decrying bicyclists on the streets. In our neighborhood, a grocery store opened and a half dozen restaurants closed. But it was the same s.p.a.ce, the same block, the same city.

I hoped that something had changed. The likeliest thing would be me.