78 



FERGUSON'S LECTURES. 



60 times as fast as the teeth of the wheel do : and conse- 

 quently, one ounce at P will balance 60 ounces hung 

 upon a tooth at q in the horizontal diameter of the 

 wheel. Then, if the diameter of the wheel F is 10 times 

 as great as the diameter of the axle G, the wheel will 

 have 10 times the velocity of the axle ; and therefore one 

 ounce P at the end of the lever A C will balance 10 

 times 60, or 600 ounces hung to the rope H which goes 

 round the axle. Lastly, if four pulleys be added, they 

 will make the velocity of the lower block K, and weight 

 W, four times less than the velocity of the axle : and this 

 being the last power in the machine, which is four times 

 as great as that gained by the axle, it makes the whole 

 power of the machine 4 times 600, or 2400. So that a 

 man who could lift one hundred weight in his arms by 

 his natural strength, would be able to raise 2400 times 

 as much by this engine. But it is here as in all other 

 mechanical cases ; for the time lost is always as much 

 as the power gained, because the velocity with which 

 the power moves will ever exceed the velocity with 

 which the weight rises, as much as the intensity of the 

 weight exceeds the intensity of the power.* 



Note 35. The screw suggested by Mr. Hunter is too valuable to be 

 passed over, while enumerating the modifications of this mechanical 

 power. The apparatus, as the annexed 

 diagram will shew, consists of a double 

 screw, or rather of two screws, one of 

 which moves within the other, and as 

 the inner screw is made finer than that 

 which turns on it, the power is very 

 considerable. In the above illustration 

 it is so made that the distance between 

 the threads of the interior screw is four 

 fifths of that of the exterior or perfo- 

 rated part, and this distance is one thir- 

 tieth of the circumference, so that the 

 weight B is capable of supporting 150 

 pounds at A. 



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