56 FERGUSON'S LECTURES. 



LECT. prop D. Its principal use is to loosen large stones in 

 ^-v-x^ the ground, or raise great weights to small heights, in 

 order to have ropes to put under them for raising them 

 higher by other machines. The parts A B and B C, 

 on different sides of the prop D, are called the arms of 

 the lever : the end A of the shorter arm AB being 

 applied to the weight intended to be raised, or to 

 the resistance to be overcome ; and the power applied 

 to the end C of the longer arm BC. 



In making experiments with this machine, the shorter 

 arm AB must be as much thicker than the longer arm 

 BC, as will be sufficient to balance it on the prop. This 

 supposed, let P represent a power,whose gravity is equal 

 to 1 ounce, and W a weight, whose gravity is equal to 12 

 ounces. Then, if the power be twelve times as far from 

 the prop as the weight is, they will exactly counterpoise ; 

 and a small addition to the power P will cause it to de- 

 scend, and raise the weight W; and the velocity with 

 which the power descends will be to the velocity with 

 which the weight rises, as 12 to 1 : that is, directly as 

 their distances from the prop; and consequently, as 

 the spaces through which they move. Hence, it is plain 

 that a man, who by his natural strength, without the 

 help of any machine, could support a hundred weight, 

 will, by the help of this lever, be enabled to support 

 twelve hundred. If the weight be less, or the power 

 greater, the prop may be placed so much farther from 

 the weight ; and then it can be raised to a proportkm- 

 ably greater height. For, universally, if the intensity 

 of the weight multiplied into its distance from the prop 

 be equal to the intensity of the power multiplied into its 

 distance from the prop, the power and weight will ex- 

 actly balance each other ; and a little addition to the 

 power will raise the weight. Tims, in the present in- 

 stance, the weight W is 12 ounces, and its distance 

 from the prop is 1 inch ; and 12 multiplied by 1 is 12; 

 tne power P is equal to 1 ounce, and its distance from 

 the prop is 12 inches, which multiplied by 1 is 12 



