It is as _a source of energy_, to be classified with heat, weight of
liquids, electricity, etc., that air in motion (as in a windmill) has a
place as a prime mover.
_Prime movers_, or receivers of power, are those pieces or combinations
of pieces of mechanism which receive motion and force directly from
some natural source of energy. The point where the mechanism belonging
to the prime mover ends and that belonging to the train for modifying
the force and motion begins may be held to include all pieces which
regulate or assist in regulating the transmission of energy from the
source of energy.
_The useful work of the prime mover_ is the energy exerted by it upon
that piece which it directly moves; and the ratio which this bears to
the energy exerted by the source of energy is the efficiency of the
prime mover.
In all prime movers the loss of energy may be divided into two
parts, one being the unavoidable effect of the circumstances under
which the machine necessarily works in the case under consideration;
the other the effect of causes which are, or may be, capable of
indefinite diminution by practical improvements. Those two parts may be
denominated as _necessary loss and waste_.
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.”
[Illustration: FIG. 460.]