70 MECHANICS. 



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

 the fulcrum, and it will consequently exert a force un- 

 der the building equal to six times its own weight. 

 If the scantling 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 

 throughout, 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 kmd, the rule appHed to 

 the first must be exactly reversed. 



COMBINATION OF LEVERS. 



A great power may be attained without the incon- 

 venience of resorting to a very long lever, by means of 

 p. 5(j Qi combination of levers. 



; :x / In Fig. 50, the small 



&; ; jk' "^^rj^^ weight P, acting as a 



'^ moving power, exerts 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. Consequently, the moving 

 power, P, acts upon the weight, W, in a twenty-seven- 

 fold degree, the former passing through a space twen- 

 ty-seven times as great as the latter. 



A combination of levers hke this is employed in self- 

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

 multiply instead of to diminish motion. The expan- 

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

 inch acts on a set of iron levers, and the motion is in- 



