Page:Popular Science Monthly Volume 15.djvu/313

 results, and in some countries, notably in Holland, quite extensively. From the best advices I have upon this topic I have it that there are in that country no less than 12,000 windmills, averaging eight horse-power each, giving a total of 96,000 horse-power.

The chief and obvious difficulties that intrude themselves against the extensive use of the wind as a motive power for general industrial uses are that in most locations it is intermittent in its action, extremely variable as to its power, and quite unreliable as to the time and duration of its manifestations.

The immense power stored up in this unfortunately unreliable agent will appear from the statement that a wind of three miles per hour travels 4·40 feet per second, and exerts a pressure of 0·32 to 0·44 pound per square foot of surface opposed to its action. A wind of twenty-five miles an hour, or what sailors would call a good stiff breeze, travels 39·67 feet per second, and exerts a pressure of from 2·208 to 3·075 pounds per square foot. The prodigious energy of a hurricane, traveling not infrequently at the rate of one hundred miles per hour, is too well known by its disastrous effects to need repetition. The power of the wind, however, save for ship-propulsion, is utilized in but few situations, its unreliability having caused it to be but very slightly esteemed in comparison with water-power and steam. Of late, however, small windmills, especially designed with superior mechanical skill, have been rapidly growing in popularity in this country, mainly for pumping water for railway and domestic purposes, an application for which these devices are excellently adapted; and I entertain no doubt that there are many situations where work is to be done that does not demand a continuous exercise of power, and where the prime consideration to be observed is the element of cheapness, where wind-power might be most advantageously employed. There are, again, extensive regions of the earth, extending for ten or more degrees north and south of the equator, where the winds blow continuously from one direction throughout the greater portion of the year—I need hardly remind you that I refer to the region of the “trade-winds,” and in which, especially along the coast-line where their influence is not disturbed by mountain ranges and other conflicting causes, the force of the wind may be relied upon with almost absolute certainty for the whole or the greater portion of the year. In such regions, therefore, Nature has supplied us with an exhaustless store of energy, capable of meeting the most extravagant demands that may be made upon it. Even the region of the temperate zones, where the winds are variable, our seashores have their strong land- and sea-breezes which for nine days out of ten may be relied upon; and even in situations where wind-power is most unreliable, as in the interior of the continents, there is a vast and valuable field open for some practical and generally applicable system by which the power of the wind, at present almost universally allowed to go to waste, may be stored up to be given out again as it