There was a gasp of astonishment, followed by silence.

"Sure, 'tis the air that's blue," hazarded the printer.

"That doesn't help much," the Forecaster said, "though perhaps it does, a little. Why is the air blue?"

The Irishman shook his head.

"Why is annything blue?" he asked.

"That's just what I'm going to tell you," the Weather Man answered, "and you want to listen carefully, boys, because the colors of the sunset depend a great deal on the weather. You can foretell weather from the sunset."

"Yo' sho' can," interrupted Dan'l. "Don't yo' remember Mammy's old rhyme:

"Evenin' red an' mornin' gray Certain signs of a beautiful day; Evenin' gray an' mornin' red, Sends a n.i.g.g.e.r wet to bed."

"All those old rhymes are fakes, though, Dan'l," declared Anton, with the importance of his newly acquired weather knowledge.

"Easy there, easy there!" warned the Forecaster. "Not so fast. A good many of those old rhymes are mighty good weather forecasts. That one is, for example."

"You mean, sir, that a red sunset and a gray sunrise really tell that the weather is going to be fine?"

"Yes, to a great extent, they do."

"Why, Mr. Levin?"

"Because they show the state of the atmosphere, boys. Rain can't fall unless there is dust. Every little drop of rain has a grain of dust in the middle. The colors of the sunset, too, are due partly to dust. Not only that, but colors of the sunset vary as the particles of dust which reflect the rays of light, are enveloped by water vapor.

"A piece of dust, without an envelope of water, is smaller than one with a little wetness around it. When more water vapor gathers around the piece of dust, the drop becomes bigger. When the sunset is red, it is a sign that it is shining on very small bits of dust, or that the condensation of water vapor into rain has not advanced very far. If, however, the sunset sky is gray, that means that the upper air is saturated, that it has all the water it can hold, and, of course, rain is likely to come soon."

"I should think, then," said Anton, "that gray in the morning would be a bad sign, too."

"It's not, though," the Forecaster replied; "the proverb is right there, as well. A gray sky in the morning means that the air is filled with water drops which are large enough to reflect light of every color.

While this is the same as the gray of evening, the processes that led to the forming of these drops is quite different. In the day the dust is heated and the forming of the droplets in the afternoon is due to cooling. In the night, the condensation is caused by loss of heat through radiation. Radiation shows that the air above must be dry.

Therefore a gray morning means a dry air above the water drops, and this means a fine day, for the droplets will soon be evaporated by the rising sun. The red morning sky declares that the dust particles have been protected from radiation by a blanket of overlying moisture, the air, therefore, is saturated to great heights and rain is probable. So you see, Anton, Mammy's rhyme is right."

"What fo' yo' talk to me against signs," declared Dan'l, putting out his chest and strutting; "Ah done told yo' them signs am pow'ful good."

"But the sunset colors, sir?" the author of the article asked. "You said they were due to dust. Just how, sir?"

"Yes, to dust, plain ordinary dust, but dust of the lightest kind," was the reply. "If you could go up in the air a hundred miles, the sky above you in the middle of the day would be jet black and the sun would shine down on you like a great bright-blue ball, without any white glare around it at all."

"Then it's a blue sun that makes a blue sky!" cried Fred.

"Don't go so fast," the Forecaster warned him. "I want you to think of the sky, first. It's a dead black, a hundred miles up. Now, at a hundred miles up, the air is so thin that there's little or no dust, but as you gradually come down and the air becomes denser, it begins to be able to buoy up some dust. Boys," he said, breaking off suddenly, "why does a stick float in water when it falls in air?"

"Because water is denser than air?" guessed Ross.

"Exactly. And why does a bar of iron sink through water and not through earth?"

"Because the particles of earth won't move aside as easily as the particles of water, I suppose."

"Not quite, but something that way. So, you see, as the air gets gradually denser it becomes gradually more able to support particles of dust, light ones at first, then heavier and heavier, until near the earth big pieces of dust can be carried in the air. You know how big some of them are when you happen to get a grain in your eye! Viscosity has a lot to do with it, too.

"The light of the sun is a bluish-white, like some of the blue stars.

White, as you remember from the rainbow, is just a mixture of all sorts of colors and the different colors are created by waves of light, some being shorter and others longer. A long wave, like the red, will pa.s.s around a tiny piece of dust, but a short wave, like the blue, will be stopped by it, and scattered, sometimes polarized, as it is called, or turned into one plane."

"I don't think I quite see that," said Anton.

"It's a little complicated," the Weather Man answered, "but maybe I can give you an idea of it. Suppose you were on a big steamboat in a choppy sea. As the steamer's length would extend over several of these waves, none of them would be big enough to make the vessel heave. If you were on that same choppy sea in a small canoe, you would be tossed in every direction. Now, if you think of the long red wave of light as a steamer and the blue as a canoe, you can see that in a ripple of small particles of dust the blue is going to be more affected than the red. In other words, the blue will be scattered. It will be diffused all over the sky and the light that comes through will be less blue."

"Then I should think the sun would look red," said Anton.

"It does," the Forecaster explained, "when there's a fog, which simply means, when there's more obstruction in the air. Sunlight is never white, as you know, it's yellow-white and the golden effect is due to dust. It's the same way at sunset. Then the rays of the sun which reach you pa.s.s through a larger amount of air, because you're looking at them from an angle, so they have to strike more grains of dust, and more of the blue rays are scattered. Then, too, when the sun, at sunset is, to you, shining obliquely on the atmosphere, it is pa.s.sing through several layers of air and these bend the rays differently."

"I still don't see," said the author of the sunset-color article, "why there should be so much pink, or rose-color, and why the clouds should generally be pink."

"There's not much pink in a clear sky," the Forecaster answered, "and as for the pink clouds, you've never seen them in the west when the sun was still above the horizon, have you?"

"No--no," said the other, "I don't think so. The pink generally comes after the sun had disappeared."

"Scientifically, of course," the Weather Man said, "the sun has gone below the horizon at least two minutes before you see it disappear.

You're looking at a sun that isn't there at all. That's due to refraction. The reason of the pink glow is that when you see it, the earth and the air for several thousand feet above you are in the shadow of the edge of the earth. The sun, therefore, is not shining on the thicker dust of the lower part of the air, but the finer dust of the upper part, the particles of which are small and more uniform in shape.

"The glow is of a rose-color because the particles are of the size to diffuse the rays of this wave-length. That's why rose colors appear in the east, before the west, and why the color lasts in the sky, which may be reflected on dust twelve miles high, after it has disappeared from the upper clouds, which are not more than eight miles high."

"'Tis the illigant hand ye are at explainin'," put in the Irishman, "but I c'n remember, when I was learnin' me trade, about thirty-four years ago, the sunsets were much finer than annything I've seen since. We don't have such sunsets now as when I was a boy."

"They were sho' brighter," agreed Dan'l. "Ah can remember when the skies used to look like they was all burning up. Ah thought the end of the world was a-comin', sho'!"

"Thirty-four years," said the Forecaster thoughtfully; "that would be in 1883, wouldn't it? Why, of course, Mike," he continued; "that was during the period of the famous Krakatoa sunsets."

"An' what's a Kraker-something sunset?" the printer asked.

"Krakatoa," the Weather Man explained, "was a volcano, near Java. In August, 1883, one of the most violent eruptions in the history of the world occurred. Half the island was blown up in the air, and, where a mountain had stood, the ocean rolled a thousand feet deep.

"The vibrations in the air were so terrific as to break windows and overturn frame houses over a hundred miles away, and the pressure wave, like some huge blast of wind, traveled round the world three times before it died down. The huge sea-waves caused by the eruption and the engulfing of the island, swept across the oceans, destroying the coasts for hundreds of leagues around. Over thirty thousand people were drowned.

"Pumice and ashes fell over the sea so thickly that within three miles of the island you could walk on them, and even five hundred miles away, the ashes formed a sc.u.m on the surface of the sea. The finer dust and the icy particles from the condensed vapor reached extreme heights in the air. These dust particles spread all round the world, completing the circuit in fifteen days.

"The sunsets were extraordinarily red, because, in the very thin air of great heights, there was an unusual amount of dust which had been forced there by the great volcanic outburst. It took three years for this dust gradually to settle into the lower air, and this made the sunsets that Pat speaks of. The great eruption of Mont Pele in 1902 created unusually beautiful sunsets in America for a couple of months afterward, but, of course, this was not to be compared to the Krakatoa eruption.

"It's curious, though, boys," he said, "that Bert, here, should have been writing this article on sunsets, because it happens that I've got something here quite important to show you."

Walking to the table, he took a large home-made portfolio from under his arm and spread it out. He untied it, threw open the cover and stepped back to let the boys look. They crowded round.

"Oh--oh!" said one. "Isn't that bully!"

The Forecaster turned over a second picture.