_Pump Jacks and Speed Jacks._--Farm pumps and speed-reducing jacks are partners in farm pumping. Force-pumps should not run faster than forty strokes per minute. Considerable power is required to move the piston when the water is drawn from a deep well and forced into an overhead tank. Jacks are manufactured which bolt directly to the pump, and there are pumps and jacks built together. A pump jack should have good, solid gearing to reduce the speed. Spur-gearing is the most satisfactory. Bevel-gears are wasteful of power when worked under heavy loads. Power to drive a pump jack is applied to a pulley at least twelve inches in diameter with a four-inch face when belting is used. If a rope power conveyor is used, then pulleys of larger diameters are required to convey the same amount of power. Only general terms may be used in describing the farm pump, because the conditions differ in each case. Generally speaking, farmers fail to appreciate the amount of power used, and they are more than likely to buy a jack that is too light. Light machinery may do the work, but it goes to pieces quicker, while a heavy jack with solid connections will operate the pump year in and year out without making trouble. For increasing or reducing either speed or power some kind of jack is needed. All farm machines have their best speed. A certain number of revolutions per minute will accomplish more and do better work than any other speed. To apply power to advantage speed jacks have been invented to adjust the inaccuracies between driver and driven. IRRIGATION BY PUMPING The annual rainfall in the United States varies in different parts of the country from a few inches to a few feet. Under natural conditions some soils get too much moisture and some too little. Irrigation is employed to supply the deficiency and drainage, either natural or artificial, carries off the excess. Irrigation and drainage belong together. Irrigation fills the soil with moisture and drainage empties it. Thus, a condition is established that supplies valuable farm plants with both air and moisture. In the drier portions of the United States, nothing of value will grow without irrigation. In the so-called humid districts deficiency of moisture at the critical time reduces the yield and destroys the profit. The value of irrigation has been demonstrated in the West, and the practice is working eastward. [Illustration: Figure 126.--Centrifugal Pump Setting. When used for irrigation, centrifugal pumps are set as close to the ground water as practical.]