Chapter IV

., on page 27. The water would be kept up in it only the thirty-four feet. The weight of a column of water of that height just balances the pressure or weight of the air. Above that height in the tube there would be a space in which there would not be any thing.

[Sidenote: Pressure of the air holds up water in the pump and mercury in the barometer.]

Quicksilver or mercury, as perhaps you know, is a fluid like water, but very much heavier. The pressure of the air, therefore, will hold up a column of this not nearly as high as the column of water it holds up. The column of mercury held up in a glass tube is not quite three feet long, while that of water is thirty-four feet.

[Illustration]

You can now understand how the instrument called a barometer is made. The object of this is to tell how heavy the air is, for the air is heavier at some times than it is at others. A glass tube, open at one end, and about three feet in length, is taken, and is filled with the mercury. Then the open end is put into a dish of mercury, as seen in the figure. There will be a space in the tube above the mercury, as represented, for the air will support by its pressure a column of only about thirty inches of mercury--six inches less than three feet, the length of the tube. A scale, divided into inches, is added, as seen in the figure; and the whole, neatly inclosed in a case, makes what we call a barometer. This means a measurer of the pressure or weight of the air.

[Sidenote: Barometer on a mountain.]

If the barometer be carried up a mountain, the mercury falls. Why is this? It is because there is less height of air pressing on the mercury than there is in the valley below, and of course it will not hold up so long a column of mercury. In the valley, as I have told you in