"It is only a steam-engine," replied she, "like that which moves a steam-boat, in which you have often been, and what you take for smoke is steam rising from boiling water, just as it does from the tea urn." "But that is real fire inside the carriage, is it not?" "Yes," replied she, "and there is real fire in the tea urn, in the shape of a red-hot heater; fire is wanted both in the tea urn and in the steam-engine, to make the water boil, for without boiling water we can have no steam; and without steam we should not be able to get on so fast, either in a boat, or on the railroad." "And then," said Willy, "we should be like the man and the pig, we should not get home to-night." "Very true, Willy, you can understand the story of the pig that would not go over the bridge, much better than how a steam-engine can move a boat, or a train of carriages." Just then a little bell went ting-a-ring-a-ring, and his Mamma told him it was to let them know that the train was going to set off; so the passengers all hastened to take their places. The train at first set off rather slowly, but then it went on faster and faster, till it got to its full speed, and Willy thought that there must be horses to make it go so fast. He looked out of the window, but the train was so long he could see neither the beginning nor the end. He saw only the houses and trees and fields, looking as if they were moving. "I know they do not," said he; "but in the railroad, I think every thing seems to be moving. And do, Papa, look, how little the cows are in that field. And are those sheep? they seem to be no bigger than lambs;--and I declare those houses," said he, pointing to them, "look almost like baby houses at the toy shop." "Those houses are really small," replied his father, "but not so very small as you suppose, for they are large enough for people to live in; every thing seen from the train when it is moving fast, appears smaller than it really is; but I will not try to explain the reason, because you could not understand it." "But, Papa," continued Willy, "the steam-engine must be stronger than horses, to be able to move the train." "Much stronger than one horse," replied he; "the engine which draws this train is one perhaps of thirty-horse power, which means that it has the power or strength of thirty horses." "But I wonder that thirty horses should be able to draw so many carriages along, such large carriages too, much bigger than our chariot, and so many people in them." "They are, indeed, a great deal larger," said his father; "for each carriage will hold eighteen persons." "But there is only room for six in this carriage," observed Willy. "True; but this is only a part of the carriage; it looks like a whole carriage inside, but if you saw it from the outside, you would find that there were two others joined close to it, to make a whole carriage."