=Quantity of Air Used.=—The next important fact that we learn from the velocity curve is the quantity of air that flows through the tube each second or minute. If we multiply the velocity with which the air escapes from the open end of the tube by the area of the end of the tube in square feet, we have the number of cubic feet of air at atmospheric pressure discharged from the tube per unit of time. The same quantity of air must be supplied to the tank in order to maintain a constant flow in the tube. In the present case that we have assumed, the tube is eight inches in diameter; therefore the cross-sectional area is 0.349 square foot. The velocity of the air as it comes out of the end of the tube is 100.4 feet per second; therefore about thirty-five cubic feet of air are discharged from the tube each second, or two thousand one hundred cubic feet per minute. This same amount must be supplied to the tank A in order to maintain the pressure constant, but when it is compressed so that it exerts a pressure of ten pounds per square inch, the two thousand one hundred cubic feet will only occupy a space of one thousand two hundred and fifty cubic feet, if its temperature does not change. This leads us to consider the effect of temperature changes. =Temperature of the Air.=—If the air is allowed to become heated by compression, as is the case in practice, we have a new set of conditions. If the air in the tank A is hot,—that is, warmer than the surrounding atmosphere,—it will by radiation cool somewhat before it enters the tube, and it will be still further cooled when it expands in the tube. Again, if its temperature falls below the temperature of the ground in which the tube is laid, it will absorb heat from the ground, and this will tend to keep up its temperature; so in practice we have very complicated relations between the temperature, pressure, and volume of the air. These relations cannot be exactly expressed by mathematical formulæ, and we will make no attempt so to express them, but will be content with saying that in practice we find that the temperature of the air in the tubes is nearly constant after the first few hundred feet, so that we can without appreciable error compute the pressures and velocities as if it were constant. Now, if the air in the tank A is hot, we must raise the pressure a little above ten pounds per square inch to obtain the velocities given on our diagram. When the air cools it contracts in volume, or, if the volume cannot change, being fixed by the limits of the containing vessel, then the pressure is reduced, so by raising the pressure in the tank A a little above ten pounds, we compensate the loss of pressure.