We have now seen that air is a gas, that it exercises pressure, that it possesses weight. We know it can be applied to many useful purposes, and that the air machines and inventions—such as the air-pump and the “Pneumatic Despatch”—are in daily use in our laboratories, our steam engines, our condensed milk manufactories, and in many other industries, and for our social benefit. Compressed air is a powerful motor for boring machinery in tunnels where steam cannot be used, even if water could be supplied, for smoke or fire would suffocate the workers. To air we owe our life and our happiness on earth.
Pneumatics, then, deals with the mechanical properties of elastic fluids represented by air. A gas is an elastic fluid, and differs very considerably, from water; for a gas will fill a large or small space with equal convenience, like the genii which came out of the bottle and obligingly retired into it again to please the fisherman. We have seen that the pressure of the air is 14⅘ per square inch at a temperature of 32°. It is not so easy to determine the pressure of air at various times as that of water. We can always tell the pressure of a column of water when we find the height of the column, as it is the weight of so many cubic inches of the liquid. But the pressure of the atmosphere per square inch at any point is equal to the weight of a vertical column of air one inch square, reaching from that point to the limit of the atmosphere above it. Still the density is not the same at all points, so we have to calculate. The average pressure at sea level is 14·7 per square inch, and sustains a column of mercury 1 square inch in thickness, 29·92, or say 30 inches high. These are the data upon which the barometer is based, as we have seen.
Fig: 56.—The “Ludion.”
In our article upon “Chemistry” we will speak more fully of the atmosphere and of its constituents, etc.
CHAPTER VI.
ABOUT WATER—HYDROSTATICS AND HYDRAULICS—LAW OF ARCHIMEDES—THE BRAMAH PRESS—THE SYPHON.
At present we will pass from Air to Water, from Pneumatics to Hydrostatics and Hydraulics. We must remember that Hydrostatics and Hydraulics are very different. The former treats of the weight and pressure of liquids when they are at rest, the latter treats of them in motion. We will now speak of the properties of Liquids, of which Water may be taken as the most familiar example.