COMBINATION OF LEVEES. 51 



be used as a lever to lift the corner of a building, then 

 the two portions, a c, c d^ will mutually balance each 

 other. If these be each a foot in length, the weight of 

 ten feet will be left to bear down the lever. The centre 

 of gravity of this portion will be at e, six feet from the 

 fulcrum, and it will consequently exert a force under the 

 building equal to six times its own w^eight. If the scant- 

 ling weigh five pounds to the foot, or fifty pounds for the 

 excess, this force will be equal to three hundred pounds. 



In the lever of the second kind, its weight operates 

 against the moving power. If it be of equal size through- 

 out, this will be equal to just one-half the weight of the 

 lever, the other half being supported by the fulcrum. 



With the lever of the third kind, the rule applied to the 

 first must be exactly reversed. 



COMBINATION OP LEVEES. 



A great power may be attained without the inconven- 

 ience of resorting to a very long lever, by means of a com- 

 yig_ 5Q hination of levers. In fig. 



50, the small weight P, act- 

 ing as a moving poAver, ex- 

 erts a three-fold force on the 

 next lever ; this, in its turn, acts in the same degree on the 

 third, which again increases the power three times. Con- 

 sequently, the moving power, P, acts upon the weight, 

 W, in a twenty-seven-fold degree, the former passing 

 through a space twenty-seven times as great as the latter. 

 A combination of levers like this is employed in self- 

 regulating stoves. It is in this case, however, used to 

 multiply instead of to diminish motion. The expansion 

 of a metallic rod by heat the hundredth part of an inch 

 acts on a set of iron levers, and the motion is increased, 

 by the time it reaches the draught-valve, to about one 

 hundred times. 



