“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. “The power of a locomotive is no doubt considerable; however, if it is able to draw with great speed a long train of cars, all heavily loaded, this is due, above all, to the preparation of the road on which it runs. Strong bars of iron, called rails, are fixed solidly on the road, all along its length, in two parallel lines, on which all the wheels of the train roll without ever running off. A light flange with which the wheels are furnished keeps the train from slipping off the rails.