=Horse-Power.=—Having shown how the quantity of air can be computed we are now in a position to estimate the horse-power of the air-compressor necessary to operate the tube. I say estimate, because we have to take into consideration the efficiency of the air-compressor, and that is not an absolutely fixed quantity: it varies with different types of machines and with their construction. When working with pressures of less than ten pounds, the friction of the machine is an important factor. The area and construction of the valves in the air-cylinders is another very important factor. If the valves are not large enough or do not open promptly, our cylinders will not be filled with air at each stroke; this will reduce the efficiency of the machine. In practice we go to a manufacturer of air-compressors and tell him how much air must be compressed per minute, and the pressure to which it must be compressed, with other conditions, and then he tells us what size of machine we shall require and the horse-power of the machine approximately. He is supposed to know the efficiency of his own machines. We may endeavor to prove his estimate by computations of our own. To give some idea of the horse-power required to supply the air needed to operate our eight-inch tube one mile long, I will say that the steam-engines of the air-compressor will have to develop in the vicinity of one hundred and twenty-five actual horse-power. From the horse-power of the steam-engine we can easily compute the coal that will be consumed under boilers of the usual type. =Efficiency.=—It will be noted that most of the power is not used primarily in moving the carriers, but to move the air through the tube. Very nearly as much power is used to keep the air flowing in the tube when no carriers are in it as when carriers are being despatched. If we should define the efficiency of a pneumatic tube as the ratio of the power consumed in moving the carriers to the power consumed in moving the carriers and the air, we should find this so-called efficiency to be very low. It is analogous to pulling the carrier with a long rope and dragging the rope on the ground. Much more power would be consumed by the rope than by the carrier. But a business man would define the efficiency of a pneumatic tube as the ratio of the cost of transporting his letters, parcels, etc., to the cost of transporting them with equal speed in any other way. Defined in this practical manner the efficiency of a pneumatic tube is high. We do not care what becomes of the power so long as it accomplishes our purpose.