148 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



40 feet in diameter, erected by a private firm on an estate in York- 

 shire, raises during a steady breeze 3,000,000 gallons of water 

 through a height of 10 feet in 24 hours. Another mill of similar 

 dimensions installed by a district council to supplement a steam 

 pumping plant has saved the council 200 tons of coal per annum 

 and has cost nothing in upkeep. It may seem strange that a power 

 showing such substantial results should not have been applied to 

 other purposes. But it may be remembered that for many years 

 after Watt's inventions the steam engine still continued to be used 

 for no other purpose than pumping. And in the case of the wind- 

 mill there are certain specific drawbacks which have militated 

 against its general adoption as a prime mover. 



The first defect is that the power hitherto obtained has been but 

 a small fraction of the available wind energy. The old corn mill 

 probably did not extract more than 5 per cent of the energy 

 theoretically obtainable from wind pressure on its sail area. The 

 modern or American type does better, perhaps to the extent of 10 

 or 12 per cent of the available energy, but not more. The figures 

 quoted above (30,000,000 foot-gallons a day) sound large, but they 

 do not represent more than 7 horsepower, as against 40 horsepower 

 which might be obtained from a thoroughly efficient wind engine of 

 equivalent area. 



In the second place, the speed of the sails at their circumference 

 is greater than the speed of the wind, and it increases without 

 limit, so that in violent storms the disruption point is reached. In 

 the old type that was avoided only by reefing; in the American 

 type it is avoided by adjusting the inclination of the vanes; but 

 in neither case is safety assured. The chance of being wrecked by 

 storms has led to a preference for small mills. 



In the third place, the variability of the power, while it does not 

 matter for pumping, is a stumbling block in the way of direct ma- 

 chine driving. So long as the mill is pumping only it does not matter 

 whether it is pumping faster or slower; but if it were driving a 

 circular saw or a loom some provision would have to be made for 

 adjusting the speed to tolerable uniformity. 



Three things are therefore desirable in a really efficient engine: 

 First, the devising of a motor which would utilize more of the wind 

 energy; secondly, the invention of means of regulating speed; and 

 thirdly, the devising of some means of storing power. All these re- 

 quirements have now been met. 



In 1881 Lord Kelvin (then Sir W. Thomson) referred to the sub- 

 ject of wind-power utilization at the meeting of the British associa- 

 tion. He pointed out that since the invention of storage batteries 

 there was no longer any need to neglect such an important natural 

 source of energy since the surplus power of a period of high winds 



