gallons of air at atmospheric pressure--then: Forcing 1,000 gallons of atmospheric air into a 1,000-gallon tank will give a working pressure of 15 pounds per square inch; 2,000 gallons, 30 pounds; 3,000 gallons, 45 pounds, and so on. Therefore, a pressure of 100 pounds in a 1,000-gallon tank (42 inches by 14 feet) would require 6,600 gallons of free atmosphere, in addition to the original 1,000 gallons, and the tank would then contain 1,000 gallons of compressed air under a working pressure of 100 pounds per square inch. A one cylinder compressor 6 inches by 6 inches, operating at a speed of 200 R.P.M. would fill this tank to a working pressure of 100 pounds in about 50 minutes. One gallon of air will deliver one gallon of water at the faucet. But the air must have the same pressure as the water, and there must be no friction. Thus, one gallon of air under a working pressure of forty-five pounds, will, theoretically, deliver one gallon of water to a height of 100 feet. But it takes three gallons of free air to make one gallon of compressed air at forty-five pounds pressure. If the lift is 100 feet, then 1,000 gallons of air under a pressure of forty-five pounds will theoretically deliver 1,000 gallons of water. Practically, the air tank would have to be loaded to a very much greater pressure to secure the 1,000 gallons of water before losing the elasticity of the compressed air. If one thousand gallons of water is needed on the farm every day, then the air pump would have to work about one hour each morning. This may not be less expensive than pumping the water directly, but it offers the advantage of water fresh from the well. Pure air pumped into the well tends to keep the water from becoming stale. [Illustration: Figure 121.--(1) Single-Gear Pump Jack. This type of jack is used for wells from 20 to 40 feet deep. (2) Double-Gear, or Multiple-Gear Pump Jack. This is a rather powerful jack designed for deep wells or for elevating water into a high water-tank.] [Illustration: Figure 122.--Post Pump Jack. This arrangement is used in factories when floor space is valuable. The wide-face driving-pulley is shown to the left.] [Illustration: Figure 123.--Three Jacks for Different Purposes. At the left is a reverse motion jack having the same speed turning either right or left. The little jack in the center is for light work at high belt speed. To the right is a powerful jack intended for slow speeds such as hoisting or elevating grain.] [Illustration: Figure 124.--Speed Jack, for reducing speed between engine and tumbling rod or to increase speed between tumbling rod and the driven machine.] [Illustration: Figure 125.--The Speed Jack on the left is used either to reduce or increase tumbling rod speed and to reverse the motion. The Speed Jack on the right transfers power either from belt to tumbling rod or reverse. It transforms high belt speed to low tumbling rod speed, or vice versa.]