"And then," said his father, "you could not crowd the ball down." "Could not a _very_ strong man?" said Nathan. "No," replied his father, "the water cannot be sensibly compressed. But now, if the tumbler contained only air, and if a ball were to be put in at the top, just large enough to fit the tumbler exactly, and if a strong man were to crowd it down with all his strength, he would, perhaps, compress the air into half the space which it occupied before." "Perhaps the tumbler would break," said Nathan. "Yes," replied his father, "and the tumbler will answer only for a supposition; but for a real experiment it would be best to have a cylinder of iron." "What is a cylinder?" said Nathan. "An iron vessel, shaped like a tumbler, only as large at the bottom as it is at the top, would be a cylinder. Now, if there was a cylinder of iron, with the inside turned perfectly true, and a brass piston fitted to it----" "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. "Why, the water above it was heavy, and pressed down upon it, and crowded it out through the hole." "Yes," said his father, "and the deeper the water, the more heavily it was pressed." "Yes, sir," said Rollo, "and the farther it spouted." "Because it was pressed down by the load of such a high column of water." "Yes, sir," said Rollo.