Questions.—How high can water be raised in a pump? Why can it not be raised higher? Tell about the experiment with a long glass tube. How high a column of mercury will the pressure of the air hold up? Explain the barometer. Explain the falling of the mercury when the barometer is carried up a mountain. How does the barometer show that the air is heavier at some times than it is at others? Why does the air seem light to us when it is heavy, and heavy when it is light? How is the barometer of use to the sailor? Tell about the storm as related by Dr. Arnot.
CHAPTER VII.
THE AIR-PUMP.
Description of the air-pump.
A great many interesting experiments about the pressure of the air can be tried with the air-pump, which you see represented here. This I will describe, so that you may understand how it works. At a, a, are two pump-barrels. In them are two pistons with valves, such as there are in common pumps, except that they are made a great deal more nicely. These pistons are worked by the handle, b. The frame-work, e e, that holds the pump-barrels, is made very strong and firm, so that the pumps may work true. There is a large plate, f, of metal, made very even and smooth. At c is a large glass vessel, close at the top, but open at the bottom. Its edge is made very smooth, so that it may fit well on the smooth plate. In the middle of the plate is a hole. This opens into a passage which leads to the bottom of the two pump-barrels.
Now you can see how the instrument works. The two pump-barrels work in the same way that a common water-pump does. With them the air is pumped out of the glass vessel by the passage which leads to them from the centre of the plate. By this means most of the air may be pumped out. If we want to let the air in after pumping it out, we loosen the screw g, for from the opening here there is a passage that leads to the hole in the centre of the plate.
Experiments.
India-rubber ball.