"What is a piston?" said Nathan.
"A piston," said his father, "is a sort of stopper, exactly fitted to the inside of a cylinder, so as to slide up and down. It is made to fit perfectly, and then it is oiled, so as to go up and down without much friction, that is, hard rubbing. There is a sort of stem coming up from the middle of the piston, called the piston rod, which is to draw up the piston, and to press it down by.
"Now," continued his father, "if a strong man had a cylinder like this, with a piston fitted to it, and a strong handle across the top of the piston rod, perhaps he might press the air into one half the space which it occupied before. That is, if the cylinder was full of air when he put the piston in, perhaps he could get the piston down half way to the bottom. Then the air would be twice as dense as it was before; that is, there would be twice as much of it in the same space as there was before. It would be twice as compact and heavy. This is called condensing air. The philosophers have ingenious instruments for condensing air.
"If, however, a man condenses air in this way, by crowding down a piston, he does not begin the condensation when the piston begins to descend. The air is condensed a great deal before he begins. All the air around us is condensed."
"How comes it condensed?" said Rollo.
"Why, you recollect that, when you bored a hole through the board in the bottom of your dam, the water spouted out."
"No, father," said Rollo, "we pulled the plug out; Jonas bored the hole."
"Well," said his father, "the water spouted out."
"Yes," said Rollo.
"What made it?" said his father.