"Do you know what the cylinder of a steam engine is?" said Forester.
"Not exactly; I don't remember it very well," replied Marco.
"Come with me, then," said Forester, "and I will show it to you."
So saying, he took Marco to the engine of the boat, and showed him, in the midst of the machinery, a large iron vessel, shaped like a hogshead, only it had straight sides. Marco could not see much more than the top of it.
"That is the cylinder," said Forester. "It is the heart of the steam engine, as I may say—the seat of its power. All the other machinery is only to aid the cylinder, and to convey the power to the point where it is wanted to do the work. Thus, the place where the steam exerts its power, and on which the whole movement of the machinery depends, is the cylinder."
Marco observed that a long iron rod, large and solid, and very bright, kept ascending and descending through the top of the cylinder, as if pushed up and drawn down again by some force within. Forester told him that that was the piston rod.
"The piston rod," said Forester, "is fastened, at its lower end, to the piston, which is a flat plate of iron, made to fit the inside of the cylinder exactly."
"First," said Forester, "the steam comes in below the piston, and drives it up; and then it is stopped from coming in below, and is forced in above, and so drives it down."
"And how does the other steam get out?" asked Marco.