The next time the Professor came, it was in a dense fog. The morning was so damp and disagreeable that we hardly expected to see him. He did not disappoint us, but seemed to have come almost before the sun was fairly up, it was so dark.

"What makes a fog?" asked Gus.

"I meant to have talked about something else, Gus," answered the Professor; "but you have chosen a subject for me. It is a very good one, too, and quite suitable to the occasion. Fogs are nothing more nor less than clouds. They usually float aloft, a mile or more, high, but sometimes drift down to the ground and lie all around us. They are so light that they rise and fall from very slight causes, when there is no wind. A brisk breeze soon drives them off."

"But what are clouds made of?" inquires May, who has become such a favorite with the Professor that she never hesitates to stop him when she wants anything explained.

"Clouds, May, are made up of small particles of water or vapor slightly chilled. When vapor or steam is hot, it can not be seen, but is invisible like the air. You have noticed the steam from a tea-kettle. Near the spout it is hidden, but a little farther off, where it has got cooled by mixing with the air, it begins to look gray, like a cloud. If the kettle be allowed to boil a long while, so that a large quantity of steam is formed, it will collect on the walls and window-panes, where, becoming thoroughly chilled, it turns again to water, the same as it was when first poured into the kettle. So it is with the clouds out-of-doors; when the sun comes out bright and hot, it dries them up, as we say; that is, it heats them so much that they become invisible. Cool air mingling with them brings them into sight again; and, if cool enough, it condenses."

"Oh dear!"

The Professor laughs. "There can be no doubt about it, May, science is full of big words. We will say that the cool wind makes the clouds heavy by squeezing them together, and sends them down in drops of rain. This is called condensing."

May rewards the Professor for his simple explanation with such a bright glance that he proceeds with an illustration.

"You have made soap-bubbles, and seen how they will float around in the air, and sometimes be wafted clear up above the trees, until they get broken, when they come down drops of water. The particles of vapor that form clouds are little bubbles, or hollow spheres filled with air. When a cold wind crushes them, they become solid, unite with one another, and fall as rain-drops. Cold water is much heavier than air; but water made hot by fire or by the sun, and turned into vapor, is lighter. In time of a fog the vapor is just warm enough to have the same weight as the air, so that it neither rises nor falls, but remains quietly near the ground."

"Professor," remarked Joe, "did you not say that when the sun came out bright and hot, it dried up the fog? and is not the fog the very thing that keeps the sun from coming out?"

"Yes, my dear; but fogs usually gather at night, and when the sun rises in the morning, he goes to work at once to heat them up and make them disappear. But when he finds them very thick, and is hindered by cold air, he may be a good part of the day in working his way through, or he may even have to go down before he is able to show himself. Generally, however, he gets help from the wind, and then the fog goes off in a hurry."

"Is there no way," asked Gus, "of knowing when the wind will spring up, and give us some clear cold weather? Ted Wynant's cousin has an ice-boat, and we are all waiting for a ride on the river."

"There is Old Probabilities," said Jack; "but he can only tell a day or two ahead, and seems rather uncertain at that, and afraid to express a decided opinion. It is a little this or a little that, a little cloudy or a little cooler, and the wind is to blow a little in nearly every direction. Most people laugh when they talk about him, as if he was not of much account, or had grown stupid in his old age. If he would only foretell a hurricane or a deluge, and bring it around, why, then we would know what he is good for."

"Such a test would be rather costly," said the Professor, smiling. "It is better to give the old gentleman a little time to establish his reliableness; for in truth he is yet very young—a mere child of eight or ten years. And considering that he undertakes to forewarn our whole country as to the coming weather, so that everybody will have time to get ready for it, we must admit that he is doing all that his age warrants."

"Where does he live?" asked Gus.

"We have been talking somewhat absurdly," replied the Professor. "Instead of a single person, there is what is called the United States Signal Service, which has been in operation eight or ten years, and comprises some two hundred or more men, scattered all over the country, from Maine to California, and from the Gulf of Mexico away out to the Northwestern lakes. The men at these various stations watch the weather very closely, and at a particular time every day send word regarding it by telegraph to the main office at Washington, where the different reports are carefully studied, and an opinion formed as to what the weather is likely to be in different sections of the country during the next twenty-four hours or more, and the result is then published in the daily newspapers and at the numerous post-offices throughout the land. The matter is yet somewhat uncertain, and occasionally mistakes are made."

"But will they ever get so that they can tell exactly every time?"

"We hope so. The warnings given are usually right, and are becoming more and more reliable every year. In 1872 it was estimated that about seventy-seven out of a hundred of them were found to be correct; more recently they have been declared accurate about ninety times in a hundred. So, you see, good progress is being made; and the Signal Service system is becoming very useful to the nation, for property and life can often be saved from destruction when the approach of a severe storm is known.

"The New York Herald has encouraged the study of the weather for many years, and its managers now send word to England by the Atlantic cable when a storm is to be expected there. They have lately sent notice of so many ugly ones, which have promptly arrived, that our English cousins are complaining of the unfair treatment of the Herald."

"Are they really so absurd?" asked Jack.

"Yes," said the Professor; "they facetiously intimate that when Providence controlled the weather they fared well enough; but that since the Herald has undertaken to run that department they have been doomed to storms, fogs, and rain. To give an instance of the faith, Jack, that the English people put in our Signal Service, there is a story told of an English lady who last autumn desired to give a lawn party. The season was an unusually rainy one, and such entertainments had, in consequence, been given up. The lady, however, sent her invitations, and calmly announced that the day she had selected would be clear. When asked how she had dared to take such a risk, she replied, 'There was no risk whatever; I had telegraphed to the man in New York.'"

The children all laughed, and it was some time before the Professor could quiet them sufficiently to add the few words that concluded his little lecture.

"The most violent storms have been found generally to whirl in circles, and are called cyclones. In some parts of the world they are very disastrous. One occurred in India in 1864 that destroyed 45,000 lives in a single day. Ten years earlier, when the English and French were at war with Russia, a storm was observed to begin in France and to be moving eastward. Timely warning was sent to the allied fleet in the Black Sea. The storm came with such terrific violence that, had it not been expected, it would probably have destroyed one of the most splendid navies that ever rode the waters, and perhaps have changed the issue of the war."