Instead of a jar-receiver, take a long-necked bottle open at both ends. If you place the hand on one of the open ends and exhaust the air, by attaching the long tube of the pump to the other you cannot remove the hand easily. Do not try to pump the air out entirely, as the suction may be too strong and draw blood. It is by the rarefaction of the air that the cupping-glass is applied to people who require bleeding. In the antiquated surgical operation of cupping, the doctor burned a few pieces of paper in small glass cups, which are then applied to the skin; the air, in getting cold, contracted and produced a partial vacuum, thus acting as the bottle did in the above experiment.
The Barometer.
Now you shall learn something about the pressure exercised by the atmospheric layer which surrounds the earth to the height of about forty miles. This is done with the aid of a very well-known instrument called the barometer.
You may construct one yourselves. Procure a glass tube closed at one end, about a yard long and one tenth of an inch in diameter. Fill it with mercury, then turn it upside down into a bowl filled with the same metal, taking care that the air does not enter the tube. The column will stop at a height between 29 and 30 inches.
This, therefore is the measure of the force of the air’s pressure, for in the upper part of the tube there is an absolute vacuum and nothing would prevent the mercury from going higher up. The weight of the air layer corresponds, therefore, to a height of nearly 30 inches of mercury.
This weight has been before stated, viz., fourteen and a half pounds, such a weight being supported by every single square inch of the globe’s surface. A marvelous pressure is thus exerted on the whole earth. In other words, the weight of the air that surrounds the earth on all sides is no less than the following enormous number of 5,184,740,000,000,000 tons.