"I should think it was," said Nathan; "it is very soft."
"It is very yielding," replied his father, "when you press it, but it is not pressed into any smaller space. It only moves away. If you have a tumbler half full of water, and press a ball down into it, you could not crowd the water into any smaller space than it occupied at first; but, as fast as the ball went down, the water would come up around the sides of the ball."
"But suppose," said Rollo, "that the ball was just big enough to fit the tumbler all around; then the water could not come up."
"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——"