CHAPTER VI

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THE BAROMETER.

Water can be raised in a pump only to a certain height, and the mistake has sometimes been made of getting the pump so long that it would not work. If it be more than about thirty-four feet from the water up to the piston, the water can not be made to go up so high. What is the reason? It is because the air, pressing on the surface of the water in the cistern or well, will raise it only to the height of thirty-four feet. It does not press hard enough to force it up any higher.

Suppose you had a glass tube over thirty-four feet long, with one end open, and used it as represented in the first experiment in