“This picture represents a locomotive. The boiler where the steam is generated, the boiling pot, in short, forms the greater part of it. It is the large cylinder that goes from one end to the other, borne on six wheels. It is built of solid iron plates, perfectly joined together with large rivets. In front the boiler terminates in a smoke-stack; behind, in a furnace, the door of which is represented as open. A man, called a stoker, is constantly occupied in filling the furnace with pit-coal, which he throws in by the shovelful; for he must keep up a very hot fire to heat the volume of water contained in the boiler and obtain steam in sufficient quantity. With an iron bar he pokes the fire, arranges it, makes it burn fast. That is not all: skilful arrangements are made to utilize the heat and warm the water quickly. From the end of the furnace start numerous copper pipes which traverse the water from one end to the other of the boiler, and terminate at the smoke-stack. You will see some in B where the picture supposes a part of the casing taken away to show the interior. The flame of the furnace runs through these pipes, themselves surrounded by water. By this means the fire is made to circulate through the very midst of the water, and so steam is obtained very quickly. [Illustration: A modern Locomotive] “Now look at the front of the locomotive. In A is seen a short cylinder closed tightly, but represented in the picture with a part of the outside removed to show what is within. There are two of these cylinders, one on the right, the other on the left of the locomotive. Inside the cylinder is an iron stopper called a piston. The steam from the boiler enters the cylinder alternately in front of and behind the piston. When the steam comes in front, what is behind escapes freely into the air by an orifice that opens of itself at the right moment. This escaping steam ceases to press on the piston, since it finds its prison open and that it can get out. We do not try to force doors when other outlets are open. So does steam act: the instant it can escape freely, it ceases to push. The entering steam, on the contrary, finds itself imprisoned. It pushes the piston, therefore, with all its strength and drives it to the other end of the cylinder. But then the rôles immediately change. The steam that hitherto has been pushing, escapes into the air and ceases to act, while on the other side a jet of steam rushes in from the boiler and begins to push in the contrary direction.” “Let me repeat it,” said Jules, “to see if I have understood it properly. Steam comes from the boiler, where it forms unceasingly. It goes into the cylinder before and behind the piston by turns. When it gets in front, that behind escapes into the air and no longer pushes; when it gets behind, that in front escapes. The piston, pushed first one way, then the other, alternately, must advance and retreat, go and come, in the cylinder. And then?”