The efficiency which a prime mover would have under given circumstances if the waste of energy were altogether prevented, and the loss reduced to necessary loss alone, is called the maximum or the theoretical efficiency under the given circumstances.
In windmills, the air, being in motion, presses against, and moves four or five radiating vanes or sails, whose surfaces are approximately helical or screw shape, their axis of rotation being parallel, or slightly inclined in a vertical plane, to the direction of the wind.
The velocity of the wind determines its pressure, and the pressure of the wind against the sails of the windmill determines the power developed by the mill. A mill of small diameter acted upon by a high pressure develops as much power as a large mill working under a lower pressure.
The mean average velocity of the wind for the entire United States is very nearly eight miles per hour. However, for large areas such as the great plains east of the Rocky Mountains, the mean average is about eleven miles per hour, and yet in certain small areas situated in the mountainous districts the mean average velocity is as low as five miles per hour. Therefore, in selecting and loading a mill, reference should be had to the wind velocity prevailing in that particular locality. In general, windmills loaded to operate in ten-mile winds can be depended upon to furnish a sufficient supply of water.
The variations in the velocity and pressure of the wind are considerable even within a brief time, and sometimes sudden and extreme. Winds of 100 miles per hour and upwards are on record. A very violent gale in Scotland registered by an excellent anemometer a pressure of 45 lbs. per square foot. During the severe storm at London, the anemometer at Lloyd’s registered a pressure of 35 lbs. to the square foot. The gauge at Girard College, Philadelphia, broke under a strain of 42 lbs. per square foot, a tornado passing at the moment within a quarter of a mile. At the Central Park Observatory, a wind was recorded of 28.5 lbs. pressure per square foot.
If the wind were to blow continuously a very small windmill would suffice to do a large quantity of work and no storage capacity would be required, but when it does blow it is “free” and experience dictates that a mill shall be erected sufficiently large to pump enough water, when the wind does blow, to last over, with the assistance of ample storage capacity.
Average hourly velocity of the wind at following stations of the U. S. Weather Bureau, given in miles per hour:
| Albany, N. Y. | 7 |
| Alpena, Mich. | 9 |
| Atlanta, Ga. | 9 |
| Atlantic City, N. J. | 10.3 |
| Augusta, Ga. | 4.2 |
| Baltimore, Md. | 6 |
| Bismarck, N. D. | 9.4 |
| Boise City, Idaho | 4.2 |
| Boston, Mass. | 10.2 |
| Brownsville, Tex. | 7.4 |
| Buffalo, N. Y. | 10 |
| Cairo, Ill. | 7.6 |
| Cape Henry, Va. | 12.7 |
| Charleston, S. C. | 8 |
| Charlotte, N. C. | 5.6 |
| Chattanooga, Tenn. | 5.5 |
| Cheyenne, Wyo. | 10.5 |
| Chicago, Ill. | 10.5 |
| Cincinnati, Ohio | 6.3 |
| Cleveland, Ohio | 9.6 |
| Columbus, Ohio | 7.6 |
| Davenport, Iowa | 8.5 |
| Denver, Colo. | 6.7 |
| Des Moines, Iowa | 7 |
| Detroit, Mich. | 8.7 |
| Dodge City, Kan. | 11.8 |
| Duluth, Minn. | 7 |
| Eastport, Me. | 0.6 |
| El Paso, Tex. | 6.3 |
| Fort Grant, Ariz. | 7 |
| Fort Sill, I. T. | 10.7 |
| Galveston, Tex. | 10.3 |
| Grand Haven, Mich. | 10.7 |
| Hatteras, N. C. | 14 |
| Helena, Mont. | 6.7 |
| Huron, S. D. | 11 |
| Indianapolis, Ind. | 6 |
| Jacksonville, Fla. | 6.7 |
| Keokuk, Iowa | 8 |
| Key West, Fla. | 9.8 |
| La Crosse, Wis. | 7.3 |
| Leavenworth, Kan. | 7.1 |
| Little Rock, Ark. | 3.6 |
| Los Angeles, Cal. | 4.7 |
| Louisville, Ky. | 7.3 |
| Lynchburg, Va. | 4 |
| Madison, Wis. | 10.2 |
| Marquette, Mich. | 8.7 |
| Memphis, Tenn. | 5.8 |
| Mobile, Ala. | 6.7 |
| Montgomery, Ala. | 5.1 |
| New Haven, Conn. | 8 |
| New Orleans, La. | 7.6 |
| North Platte, Neb. | 10.3 |
| Olympia, Wash. | 3.8 |
| Omaha, Neb. | 8.5 |
| Oswego, N. Y. | 9.6 |
| Pensacola, Fla. | 8.2 |
| Philadelphia, Pa. | 10 |
| Pittsburg, Pa. | 6 |
| Portland, Me. | 8 |
| Portland, Ore. | 5.3 |
| Prescott, Ariz. | 6.5 |
| Red Bluff, Cal. | 7 |
| Roseburg, Ore. | 5.3 |
| Sacramento, Cal. | 6.7 |
| St. Louis, Mo. | 10.3 |
| St. Paul, Minn. | 7.6 |
| St. Vincent, Minn. | 9.4 |
| Salt Lake City, Utah | 5.3 |
| Sandy Hook, N. J. | 14.5 |
| San Diego, Cal. | 5.6 |
| San Francisco, Cal. | 9.4 |
| Savannah, Ga. | 7 |
| Shreveport, La. | 5.6 |
| Spokane Falls, Wash. | 4.7 |
| Springfield, Ill. | 8.7 |
| Vicksburg, Miss. | 5.8 |
| Washington, D. C. | 6.5 |
| Yuma, Ariz. | 6 |
| Yankton, S. D. | 9 |
Note.—Windmills are erected to be operated by the lightest winds. A wind which will carry off smoke will move a windmill; and the absence of a wind of this force means a perfect calm. Mr. Corcoran says: “My experience of thirty years teaches that a calm has seldom, if ever, held sway in this part of the world for a longer period than three days. Consequently, with a tank to hold a three days’ supply, it becomes possible to pass over any number of calms.”