EXPANSION OF GASES.

8. One of the chief characteristics of any gas is its expansive property. To calculate the expansion of any volume of air, the starting-point must always be taken at 0° on Fahrenheit's scale; for air at that temperature will expand 49 of its volume for every degree of heat added. Therefore 459 cubic feet of air at 0° will become 460 cubic feet at 1° F.

Careful experiments show that 459 cubic feet of air at 0° F. weigh 39.76 pounds, when the pressure is 30 inches of mercury of the density due to 32°, a pressure equal to 14.7 pounds per square inch; but, when the pressure is one inch, it weighs only part of this, or 1.3253 pounds. To find the weight of a cubic foot of air at any temperature or height of the barometer, let

then

B =

(1)

height of the barometer in inches,

t = temperature by Fahrenheit's thermometer;

Problem. What is the weight of a cubic foot of air, the temperature of which is 96°, under a barometric pressure of 29.5 inches of mercury?

By substituting 29.5 in formula 1 for B, and 96° for

t, and then performing the operations indicated, we have

as the weight of a cubic foot of air under the above conditions.

The following table has been made out to facilitate calculations. It gives the weight of 100 cubic feet of air in pounds at different barometrical pressures.

9. As we have seen, air has weight, and therefore becomes subject to the "physical laws" that govern liquids and falling bodies; i.e., air is acted on by gravity in the same manner as a solid.

Let h = the distance fallen through in feet.

v=

=

the velocity acquired at the end of the fall, in feet per second.

g the distance in feet which an unresisted gravitating body falls in the first second of time; which distance has been found by experiment to be 162 feet near the earth's surface.

Since a body falls 16.08′ in one second, it gains a velocity of 32.16' at the end of the first second: hence we have

In this equation h represents the necessary height, in feet, of a vertical air-column which will produce by its weight a velocity equal to v. If this velocity be represented in feet per minute, we shall have

These formulæ (2), (3), and (4) are only theoreti cally true as regards air in mines, the pressure or head of air-column required being from ten to twenty times as much in order to overcome the friction and resistance of airways, etc., in underground workings; and, were it not for these resistances, very small pressure would suffice to produce great velocity.

CHAPTER II.

NATURAL VENTILATION.

10. MOTION in air is caused by pressure, or difference of pressure. When air becomes heated, it ascends, because it assumes a larger volume;

E

and, as the same volume of cold air is heavier, it pushes the warmer A up, or out of its place. This phenomenon in the open air gives rise to winds and breezes, which vary in intensity, according as the cool air takes the place of the warm air, rapidly or slowly. Suppose we have two shafts (connected, as in the above figure) of equal depths, the air in them having

B