\VIND-POWEE WINDMILLS. ] 



APPLIED MECHANICS. 



825 



circle in which that point revolves is 94 feet, and the 

 number of revolutions made per minute is therefore 

 W 1 - about 7. 



If, now, we calculate the speed of the extremities of 

 the arms, we find that it is 1,320 feet per minute, or 

 about 15 miles per hour ; three times that of the wind, 

 which we have assumed as 5 miles per hour. Did we 

 assume a wind of greater velocity, we should have to 

 take into account the self-regulating arrangement, which 

 diminishes the amount of surface exposed, and therefore 

 prevents the mill from attaining so much increase of speed 

 as it would without regulation. Under ordinary circum- 

 stances, the speed of the outer extremities of the arms 

 ranges from 20 to 30 miles per hour. We may assume 

 30 miles per hour when the wind blows at 10 miles, with 

 a pressure of about ^ Ib. on the square foot. The total 

 surface of the sails unfurled, in a mill 60 feet dia- 

 meter, is 1,250 square feet ; we may suppose half lost 

 by furling ; leaving 625 effective. As the surface is set 

 obliquely to the wind, the pressure in the direction of 

 motion would be reduced from Ib. to about i Ib., as a 

 mean over the whole of the arms, giving a total pressure 

 in the direction of motion of about 90 Ibs. The mean 

 velocity of the arms is half that of the extreme, 15 miles 

 per hour, or 1, 320 feet per minute. We have, therefore, 90 

 Ibs. moving at 1,320 feet per minute, which is equivalent 

 to a force of 90 X 1,320 = 118,800 Ibs., moving at 1 

 foot PIT minute. A horse-power is reckoned as equi- 

 valent to 33,000 Ibs., moved 1 foot per minute ; there- 

 fore, the power of the mill we have reckoned is about 

 3| horse-power. 



"When we double the diameter of a mill, we quadruple 

 its power, for we quadruple its effective surface. The 

 areas of circles are proportional to the squares of their 

 diameters ; and as the similar parts of the areas are 

 occupied by sails, they are also as the squares of the 

 diameters. 



It is not at all an easy matter to estimate the powers 

 of windmills. The proper guide as to power, velocity, 

 and construction, is experience. Some of the works of 

 Smeaton contain much valuable information respecting 

 this branch of Practical Mechanics ; and to these we 

 must refer such of our readers as require a more full 

 discussion of the subject than our limits permit us to offer. 

 As a force applied to the movement of machinery, 

 wind has few advantages, except its little cost after the 

 first outlay for a windmill has been made. It is chiefly 

 available in flat countries, where there is no opportunity 

 of obtaining the preferable power of water, and where 

 there is little interruption to the aerial currents. In 

 hilly countries windmills are often subject to derange- 

 ment, from the excessive force of the gusts of wind that 

 occur in such regions. In tropical countries, particularly 

 islands and places near the sea-shore, the daily occur- 

 rence of the land and sea-breezes, occasioned by the 

 action of the solar heat on the land, provides a certain 

 amount of wind-power, which may be almost always 

 depended on. But in these countries, on the other 

 hand, there often occur tornadoes, or hurricanes of ex- 

 treme violence, that sweep away almost everything that 

 may oppose their progress ; and thus frequently destroy 

 windmills, and occasion renewed outlay in their re- con- 

 struction. The principal use to which windmills are 

 devoted in temperate climates, is for grinding corn ; in 

 tropical climates, such as the West Indian Islands, they 

 are employed for driving sugar-cane mills. In fenny 

 and marshy countries, such as Holland, or some of the 

 eastern counties of England, they are used for drainage, 

 either by working pumps or turning a wheel contrived 

 for lifting the drainage water from the surface of the 

 ground into canals at a higher level, by which it is carried 

 off into the sea. In all situations, however, where the 

 cost of fuel is not extravagantly great, steam-power is 

 gradually superseding that of wind, because its certainty 

 of action more than repays the cost of its production. 

 Whole districts, the drainage of which is dependent on 

 wind-power, may frequently remain many weeks under 

 water from the prevalence of calm weather, and the 

 agricultural operations of the season may be so seriously 



MIL. I. 



interfered with, that whole crops are lost, or become 

 immensely deteriorated. In sugar-growing countries, 

 again, the derangement of wind-machinery by a hurricane 

 or tempest, may occur at the season when the sugar- 

 canes have to be crushed ; and the loss of a few days in 

 crushing the canes, may seriously damage the sugar in 

 respect of quantity as well as quality. Upon the whole, 

 then, whenever the cost of fuel is not excessive, it is not 

 advisable to incur the outlay of extensive worka for 

 securing wind-power. A very small steam-engine, kept 

 constantly in operation, is far more effective than a wind- 

 mill of much greater power, because the latter is so 

 variable and uncertain in its action. The only operations 

 suited to wind-power, are such as need not necessarily 

 be completed at certain periods, but may be conducted 

 occasionally as the wind may serve. Nor should the 

 machinery driven by wind require very nice regularity 

 in its action ; for, notwithstanding all the ingenious 

 arrangements for equalising the wind-force, it is still 

 unsteady at the best. 



Every part exposed to the wind should be greatly in 

 excess of the strength required to resist the average 

 strain to which it may be exposed. The tempest of an 

 hour nay, a momentary gust may frequently destroy a 

 windmill that has stood under ordinary winds for years. 

 As a safeguard against too much strain, the windmill 

 should always be left free to revolve, even if the ma- 

 chinery which it drives be thrown out of gear. The 

 shaft or axis of the mill generally carries a large wheel, 

 to which is fitted a strap of iron loaded so as to press on 

 its circumference, and act as a friction-break either to 

 hold the mill fast for purposes of repair during light 

 winds, or to check its velocity when the winds are too 

 strong for the work required. 



WATER POWER. The movements of water are 

 much more serviceable for the purposes of power, and 

 steady in their operation, than those of air. In level 

 countries, where the streams are slow and languid in their 

 flow, this power is not attainable ; but in hilly countries, 

 where the rivers and streams fall frequently from a high 

 level to a lower, water-power is easily obtained, and is 

 most advantageous as a steady, inexpensive prime 

 mover. The most common way of employing water- 

 power is to cause the current to act on the circumference 

 of a large wheel, so as to give it a rotatory motion, 

 which is communicated, by means of shafts and wheel- 

 work, to the machinery required to be driven. Such 

 water-mills are generally used for grinding or thrashing 

 corn, crushing bones for manure, raising water to irrigate 

 laud ; in mining districts, for crushing or otherwise 

 operating on the ores ; and in manufacturing districts, 

 for working cotton, woollen, or flax machinery. Water- 

 wheels are of three kinds, named according to the 

 mode in which the water-current is made to act upon 

 them : 



1. Undershot, when the wheel is fixed over a stream 

 with inconsiderable fall, but considerable velocity. 



2. Overshot, when the fall of the water is so great 

 that the stream may be directed upon the upper part .of 

 the wheel. 



3. Breast-wheels, when the stream can be directed on 

 or near the middle or breast of the wheel. 



1. The Undershot-wheel may be best understood by 

 conceiving the action of the paddles of a steam-vessel 

 reversed ; that is to say, while in a steam-vessel the 

 paddles are caused to revolve, and were the vessel fixed 

 would produce a current in the water by their revolution, 

 in the case of the undershot- wheel, the natural current of 

 the water pressing on the floats immersed in it, causes 

 the wheel to revolve. It is sufficiently clear that the 

 power derived from this arrangement depends upon the 

 intensity of the pressure which the water exerts on the 

 floats, and the amount of surface pressed upon. If we 

 suppose the wheel at rest, and its float standing vertically 

 in the water, we may easily compute the pressure on 

 every square foot of its surface by ascertaining the speed 

 at which the water flows against it. This pressure, like 

 that of the wind, is proportional to the square of the 

 velocity of current ; for by doubling the velocity, we 



