The longer a chimney is, the more perfect is its draft, provided the fire is great enough to heat the column of air in it, because the tendency of the smoke to draw upwards is in proportion to the difference of weight between the heated air in a chimney and an equal column of external air.

The first we hear of chimneys, for the escape of the smoke from a fire or furnace, is in the middle ages.

[Ill.u.s.tration: FLOATING DRY DOCK "DEWEY"

This dry dock, which is capable of floating the largest battleship, was towed from Sparrow's Point, Maryland, to Olangapo, Philippine Islands, a voyage of 13,000 miles. In operation, the dock is sunk by admitting water into its tanks until the ship can be floated in. The water is then pumped out and the dock with the ship inside rises to the proper level as shown.]

[Ill.u.s.tration: U. S. BATTLESHIP "MISSISSIPPI" IN DRY DOCK AT LEAGUE ISLAND]

What are Dry Docks Like?

Although divers are able to go down under the water to examine the bottom of a ship while it is afloat, it is usually necessary to have it up on dry land when thorough inspections or repairs have to be made. So a berth something like a huge box stall in a stable is built, with the part where a horse would stand in the stall full of water, and a door, either made like swinging gates opening in the middle, or a caisson which is operated up and down like a window, at the end. The ship is floated into the dock and then after the door is shut to prevent any more coming in, all of the water is pumped out until the vessel rests on a lot of great big wooden blocks and supporting props with which the bottom and sides of the dock are lined. Supports are also placed between the vessel and each side of the dock. Then, when the work has been finished, and the ship is ready to go to sea, water is let back either by pumping it in or else by gradually opening the door at the end, and the vessel is able to float out into the river or harbor again.

Although all of the navy yards and some private corporations in this country have docks of this kind, they are not of as much importance here as in England, where they are used, without pumping out the water, for the loading and unloading of vessels, because of the very great rise and fall of the tides there straining and otherwise damaging ships tied up to ordinary docks.

There are nine important navy yards in the United States, located at Brooklyn, N. Y.; Boston, Ma.s.s.; Portsmouth, N. H.; Philadelphia, Pa.; Portsmouth, Va.; Mare Island, Cal.; New London, Conn.; Pensacola, Fla.; Washington, D. C., and Port Orchard, Wash.

There is another kind of dry dock, called "floating docks," which float on the surface of the water and may be sunk sufficiently to allow of a vessel being floated into them, and then raised again by pumping the water out of the tanks around the sides. They are usually built of iron, with water-tight compartments, and not closed in at either end. They are sunk to the required depth by the admission of water into so many of the compartments, till the vessel to be docked can float easily above the bottom of the dock, and then they are raised by pumping out the water until the ship can be propped up as in the land dry dock.

Why does a Lightning Bug Light Her Light?

The lightning bugs or fireflies which are seen so often on summer evenings in the country and among the trees in the parks of the city, are similar to the species of beetle called the glowworm in Great Britain, although the glowworm there does not give as much light as the firefly in America.

In reality it is only the female which is the lightning bug, for the male is not equipped with any lighting power. He has the bad habit of going out nights, and so the female has had to make use of her ability to make part of her body shine with a sort of a phosphorus green light in order to show him the way home, very much as a dweller in a poorly-lighted street keeps a light in the window or on the porch to guide visitors or the late home-comer to the proper house. She seems to possess the power of moderating or increasing the light at will.

The most brilliant fireflies are found only in the warmer regions of the world. The ordinary firefly to which we are accustomed gives off a very much brighter light if placed in warm water. Fine print may be read by the light of one kind which is found in the West Indies; in Cuba the ladies have a fashion of imprisoning them in bits of netting or lace of a fine texture and wearing them as dress ornaments, and in Hayti they are used to give light for domestic purposes, eight or ten confined in a vial emitting sufficient light to enable a person to write.

The Story in the Making of a Picture[10]

Let us suppose, for the purposes of explanation, that as far as _seeing_ goes, any object is made up of countless infinitesimal points of light, and that the business of the eye is to gather them in and spread them out at the back of the eye in exactly the same relation they bore to each other on the object. The points of light, so duplicated, would thus form the image of the object.

The camera works very much the same way. The lens at the front of the camera is the eye, and the plate or film at the back of the camera corresponds to the back of the eye. The lens collects all the points of light of the object we wish to photograph, and directs them to the plate or film in such fashion that they occupy exactly the same relative position that they did before. An image of the object is formed.

Now if we could look inside the camera and the image were visible, we would see that it was upside down. The reason for this is very simple, as the accompanying diagram shows. The ray of light from "A" at the bottom of the object pa.s.ses through the lens at an angle, and continues in a straight line until interrupted by the film or plate. It started at the bottom of the object and ended at the top of the image. The position of all the points of light is just reversed, although their relative position remains the same.

[Ill.u.s.tration: SHOWING INVERSION OF THE IMAGE]

"Then here," you say, "is where your a.n.a.logy between the camera and the eye falls down."

Not at all. It is true that we do not see things upside down, but this is because of mental readjustment during the pa.s.sage of the impressions from the eye to the brain.

Now let us suppose that we have our camera loaded with film, and that mother has succeeded in keeping the baby quiet long enough for us to uncover the lens for an instant and let the points of light through to the film. The next question is, how are we going to make the resulting image permanent. We know that it is there, but in its present state it is not going to do us a great deal of good. In fact, if we should peek in the back of the camera, and to do so would ruin the exposure, we could not even see it.

But let us go back a bit. We ought to know a little something about the composition of this film on which the image has been projected.

In brief, film is a cellulose base coated with silver bromide and gelatine. If we were using a plate the only difference would be that instead of cellulose as a base we would have a sheet of gla.s.s. The gelatine is there to afford lodgment to this sensitized silver. The silver, being sensitive to the action of light, is there to record the image. As soon as one of these silver particles has been touched by light, it becomes imbued with the power of holding whatever the lens has transmitted to it. The image was formed, we remember, by points of light grouped in the same relative positions as the points of light of the object we were photographing. Consequently it is only those silver particles within the image-forming area that are affected, because that is where the light struck.

The lens, then, gathered in the points of light and dispersed them on the film so as to form an image. The silver particles held this image, but not visibly--it is a latent image, and it is the purpose of development to bring it out.

It is the particular business of a chemical called "pyro" to release this latent image. When attacked by pyro, those silver bromide particles which have been affected by light--and only those--change to black metallic silver. After all the silver bromide particles, the ones that held the image, have been transformed into metallic silver, another chemical called "hypo" effectively disposes of all the silver bromide that was not affected by light. Now only the image-forming silver bromide particles remain, and these have been transformed to metallic silver. The result is a permanent image--a negative.

But it _is_ a negative, so called because everything in it is reversed--not only from left to right, but in the details of the image.

Mother's dark blue gown looks light, for example, and baby's white dress, dark.

To get our picture as it should be, we must place the negative in contact with a sheet of paper coated with a gelatine containing silver.

This emulsion, as the coating is called, is, as we might readily infer from the presence of the silver, sensitive to the action of light in much the same manner as was the original film. We place the negative and paper in contact, then, in what is called a printing frame, so that light may shine through the negative and impress the image on the sensitive paper. It is obvious that the light parts of the negative will let through the most light, and that consequently the silver emulsion on the paper underneath will be most blackened, while the dark parts will hold back the light and the emulsion on the paper underneath will be less affected. In other words, the very faults that we noted in the negative, from a picture point of view, automatically right themselves.

Mother's dress looks dark and baby's dress white--just as the lens saw it.

We then have the picture in its finished form.

The story of the making of the camera is as interesting as that of the making of the pictures by the camera.

Back in 1732, J. H. Schulze discovered that chloride of silver was darkened by light and all unwittingly became the father of photography.

In 1737, h.e.l.lot, of Paris, stumbled on the fact that characters written with a pen dipped in a solution of silver nitrate would be invisible, until exposure to light, when they would blacken and become perfectly legible. However, it was not until early in the nineteenth century that these two discoveries were put to any practical use, as far as photography was concerned.

People of an artistic turn of mind had been in the habit of making what were called "silhouettes." The sitter was so posed that the light from a lamp threw the profile of his face in sharp shadow against a white screen. It was then easy enough to obtain a fairly accurate silhouette, by either outlining the profile or cutting it out from the screen.

It occurred to a man by the name of Wedgwood that this profile might be printed on the screen by using paper treated with silver nitrate, and he not only succeeded in accomplishing this, but also in perfecting what was then called the "camera obscura," the forerunner of the kodak of today. The camera obscura consisted of a box with a lens at one end and a ground gla.s.s at the other, just like a modern camera. It was used by artists who found that by observing the picture on the ground gla.s.s they could draw it more easily. Wedgwood tried to make pictures by subst.i.tuting his prepared paper for the ground gla.s.s, but the paper was too insensitive to obtain any result. Sir Humphrey Davy, continuing Wedgwood's experiments, and using chloride of silver instead of nitrate, succeeded in making photographs through a microscope, by using sunlight. These were the first pictures made by means of a lens on a photographic material. But none of these pictures were permanent, and it was not until 1839 that Sir John Herschel found that "hypo," which he had himself discovered in 1819, would enable him to "fix" the picture and make it permanent.

At about this time, Daguerre announced discoveries that gave photography at least a momentary impetus, but the Daguerre process did not long survive, as it was slow, costly and troublesome. The daguerreotype was made on a thin sheet of copper, silver plated on one side, polished to a high degree of brilliancy, and made sensitive by exposing it to the fumes of iodine. The first daguerreotype made in America, that of Miss Catherine Draper, was exposed for six minutes in strong sunlight, and the face of the sitter thickly powdered, to facilitate the exposure. An exposure today with a modern camera, under similar conditions, could be made in 1/1000 of a second.

[Ill.u.s.tration: ARTOTYPE COPY OF THE EARLIEST SUNLIGHT PICTURE OF A HUMAN FACE

Miss Dorothy Catherine Draper, taken by her brother, Prof. John W.

Draper, M.D., LL.D., in 1840.]

It was impossible, of course, to find many sitters as patient as Miss Draper--try keeping perfectly quiet for even a minute if you would know why Miss Draper should be ranked as a photographic martyr--and many experiments were made in an attempt to materially shorten the time of exposure. The only real solution, of course, was to find some method where the light had to do only a little of the work, leaving the production of the image itself to chemical action.

[Ill.u.s.tration: OLD-FASHIONED PHOTOGRAPHIC EQUIPMENT]

The first great step in this direction was taken by Fox Talbot in 1841.

He found, that if he prepared a sheet of paper with silver iodide and exposed it in the camera, he got only a very faint image, but if, after exposure, he washed over the paper with a solution of silver nitrate and gallic acid, the faint image was built up into a strong picture. And not only was Fox Talbot the first to develop a faint or invisible image; he was also the first to make a negative and use it for printing.

[Ill.u.s.tration: THE FIRST KODAK (1888), SHOWING ROLL HOLDER AND ROLL FILM FOR 100 EXPOSURES]

[Ill.u.s.tration: THE FIRST DAYLIGHT LOADING METHOD]

[Ill.u.s.tration: THE FIRST "FOLDING KODAK" FITTED FOR PLATES OR ROLL FILM]

[Ill.u.s.tration: "DOPE" BARREL]

In spite of all these advances, photography was almost exclusively a studio proposition, when, in 1880, experiments were begun which were to result in photography that could be universally enjoyed--photography as we know it today. Of course there were amateurs even in those early photographic days, but they were few and far between. There was something about the bulk and weight of the old-time photographic outfit that failed to beget general enthusiasm.