From Table V it will be seen also that a wind velocity of fifteen miles per hour develops a power three times as great as an eight-mile wind, and a twenty-mile wind is twice as powerful as a fifteen-mile, or six times that of an eight-mile. Hence, a small increase in velocity greatly increases the power of the windmill, while a low velocity gives but little working force.
From Table VI it is seen that a twenty-five mile wind gives six times as much power as a ten-mile wind, but really gives twenty-six times the net efficient power of the ten-mile wind, therefore it will not be proper to calculate on using a power windmill in as low a velocity as ten miles.
From Table VII it is seen that the net efficient result is six times as great in a fifteen-mile wind as in a ten-mile wind, and sixteen times greater in a twenty-mile wind than in a ten-mile wind. Therefore, power windmills give best results when working in fifteen to twenty-five mile winds. A 12-foot power windmill working in a fifteen-mile wind will do more work than an average horse, and when working in a twenty-mile wind will do more work than two average horses.
Example.—A person in Atlanta, Ga. uses 2,600 gallons of water daily. He has a well in which the water stands 30 feet from ground level. To obtain pressure, the water is to be elevated into a tank 50 feet above ground. 2,600 ÷ 8 = 325 gallons to be pumped hourly when windmill works.
Average wind velocity at Atlanta is 9 miles per hour, answering to coefficient 1.4 in Table II, and total water lift is 30 + 50 = 80 feet. 325 × 1.4 × 80 = 36,400 gallons.
If first estimate of 2,600 gallons daily was liberal, so that for instance 2,400 gallons would be sufficient, Table I shows that a 10-foot mill can be used, but to keep on the safe side, choose a 12-foot mill. 325 gallons hourly gives us in Table III 31⁄4-inches cylinder with 2-inches discharge pipe as proper sizes. If the 10-foot mill is chosen take the 3-inch cylinder.
A 14-foot windmill working in a fifteen-mile wind will do more work than two average horses, and when working in a twenty-mile wind will do more work than four good horses, while in a twenty-five mile wind it will do more work than six good horses.
Giving the above tables a practical application, a little thought will disclose what a wealth of power stands unappropriated and ready at hand to do many of the drudgeries of work for which large expenditures are annually made.
The uses of power windmills are so well understood that it seems out of place to elaborate upon them; the brief space allowed to giving information as to the power of this class of mills when working in different wind velocities, is best expressed in tabular form, Table VI.