Forests of Maine / Marco Paul's Adventures in Pursuit of Knowledge

by Jacob Abbott

"There are two ways of getting rid of the steam that is below the piston when the piston is coming down," said Forester. "One way is, to open a passage to let it out into the air. On this plan, when the piston has been driven up, the steam is cut off from coming in below the piston, and is admitted above. At the same instant, the passage is opened to let the lower steam out. Of course, the steam that comes in above, drives the piston down, and forces the steam that is below, out into the air. They generally have a pipe to convey it away, and as the piston goes up and down, the steam comes out in puffs, as you saw it in that mill."

"Yes," said Marco, "I understand that; and now what is the other way?"

"That kind of engine is called a high pressure engine," said Forester, "because the piston, in coming down, has to drive out the steam from below, against the pressure of the atmosphere; for the atmosphere above passes into the pipe, and resists the movement of the steam in coming out. It requires a greater force of steam to work the piston on this plan than it does upon the other."

"What is the plan of the other?" asked Marco.

"On the other plan," said Forester, "the steam under the piston is condensed, that is, turned suddenly into water; and this leaves a vacancy or void below the piston, so that the piston can be forced down much more easily than if it had to drive the steam out before it, against the pressure of the atmosphere."

Forester was going on to explain to Marco how it was that the steam was condensed in the cylinder, when the conversation was suddenly interrupted by the sound of the engine bell, which was the signal for the engine to stop. The thumping sound of the machinery and of the paddle wheels accordingly ceased, and the boat began to move more slowly. Presently, the bell sounded once more, and the piston rod slowly rose out of the cylinder, and then slowly descended again.

"They are going very slowly," said Marco.

"Yes," replied Forester, "the water is low, and I suppose that the channel is narrow."

Just at this moment, they perceived a strange sensation, as if the steamboat had been suddenly pushed backwards. Marco was startled. He did not know what it meant.