The Story-book of Science

by Jean-Henri Fabre

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?”

“The piston is in the form of a solid iron rod that enters the cylinder through a hole pierced in the middle of one of the ends, and just large enough to give free passage to the rod, without letting the steam escape. This rod is bound to another iron piece called a crank, and finally the crank is attached to the neighboring wheel. In the picture all these things can easily be seen. The piston, advancing and retreating in turn in the cylinder, pushes the crank forward and back, and the crank thus makes the great wheel turn. On the other side of the locomotive the same things are taking place by means of a second cylinder. Then the two great wheels turn at the same time and the locomotive moves forward.”

“It isn’t so hard as I thought,” Jules remarked. “Steam pushes the piston, the piston pushes the crank, the crank pushes the wheel, and the engine moves.”

“After acting on the piston, the steam enters the same chimney that the smoke comes out of. So you can see this smoke-stack sometimes throwing out white puffs, sometimes black. These latter are smoke coming from the furnace through the tubes that go through the water; the others come from the steam thrown out of the cylinders after each stroke of the piston. These white puffs, in rushing violently from the cylinder to the smoke-stack after acting on the piston, make the noise of the engine as it moves.”

“I know: pouf! pouf! pouf!” exclaimed Emile.

“The locomotive carries with it a supply of coal to feed the fire, and a supply of water to renew the contents of the boiler as fast as evaporation may require. These supplies are carried in the tender; that is to say, in the vehicle that comes immediately behind the locomotive. On the tender are the stoker, who tends the furnace, and the engineer, who controls the passage of the steam into the cylinders.”

“The man in the picture is the engineer?” Emile asked.

“He is the engineer. He holds his hand on the throttle, which allows the steam from the boiler to enter the cylinders in greater or less quantity, according to the speed he wishes to obtain. By one movement of the throttle, the steam is cut off from the cylinders and the engine stops; by another movement the steam is admitted and the locomotive moves, slowly or rapidly at will.