AS their uncle finished speaking, the postman came with a letter. A friend advised Uncle Paul to go to town on pressing business, and he wished to take advantage of the occasion to give his nephews the diversion of a little journey. He had Jules and Emile dressed in their Sunday clothes, and they set out to wait for the train at the neighboring station. At the station Uncle Paul went up to a grating behind which was a very busy man, and through a wicket he handed him some money. In exchange the busy man gave him three pieces of cardboard. Uncle Paul presented these pieces of cardboard to a man who guarded the entrance to a room. The man looked and let them enter.

Here they are in what is called the waiting-room. Emile and Jules open their eyes wide and say nothing. Soon they hear steam hissing. The train arrives. At its head is the locomotive, which slackens its speed so as to stop a moment. Through the window of the waiting-room Jules sees the people passing. Something preoccupies him: he is trying to understand how the heavy machine moves, what turns its wheels, which seem to be pushed by an iron bar.

They enter the railway car, the steam hisses, the train starts, and they are off. After a moment, when full speed had been gained: “Uncle Paul,” said Emile, “see how the trees run, dance, and whirl around!” His uncle made him a sign to be silent. He had two reasons for this: first, Emile had just made a foolish remark, and, secondly, his uncle did not choose to notice the giddy-pate’s self-betrayal in public.

Besides, Uncle Paul is not very communicative when traveling; he prefers to maintain a discreet reserve and keep silence. There are people whom you have never seen before, and perhaps will never see again, who immediately become very familiar with their traveling companions. Rather than hold their tongues they would talk to themselves. Uncle Paul does not like such people; he considers them weak-minded.

By evening the three travelers had returned, all much pleased with their trip. Uncle Paul had brought to a favorable conclusion his business in town. Emile and Jules each came back with an idea. When they had done honor to the excellent supper Mother Ambroisine had prepared on purpose to wind up the holiday with a little treat, Jules was the first to impart his idea to his uncle.

“Of all that I saw to-day,” he began, “what struck me most was the engine at the head of the train, the locomotive that draws the long string of cars. How do they make it move? I looked well, but could not find out. It looks as if it went by itself, like a great beast on the gallop.”

“It does not go by itself,” replied his uncle; “it is steam that puts it in motion. Let us, then, first learn what steam is and what its power.

“When water is put on the fire, it first gets hot, then begins to boil, sending off vapor, which is dissipated in the air. If the boiling continues some time, it ends with there being nothing in the pot; all the water has disappeared.”

“That is what happened to Mother Ambroisine day before yesterday,” put in Emile. “She was boiling some potatoes, and having neglected to look into the pot for some time, she found them without a drop of water, half burnt. She had to begin all over again. Mother Ambroisine was not pleased.”

“By heat,” continued Uncle Paul, “water becomes invisible, intangible, as subtle as air. That is what is called vapor.”

“You told us that the moisture in the air, the cause of fogs and clouds, is also vapor.” This from Claire.

“Yes, that is vapor, but vapor formed only by ............