THE ELEMENTS OF MACHINERY. 



95 



"Weigh t, and 

 the length of 

 the arms of a 

 lever being 

 given, how wa 

 find the equiv- 

 alent power ? 



What are ex- 

 amples of le- 

 VTS of the 

 aecond class ? 



of a lever are given, the power requisite to 

 balance the weight may be ascertained, by 

 dividing the product of the weight multiplied 

 into its distance from the fulcrum, by the dis- 

 tance of the power from the fulcrum. 



204. Cork, or lemon-squeezera, Fig. 63, are examples of 

 the levers of the second class, which have the fulcrum at one 

 end, and the weight, or resistance to be overcome, between 

 the fulcrum and the power. An oar is a lever of the second 

 (daas, in which the reaction of the water against the blade is the fulcrum, the 

 Fig. 63. ^o^t tbe weight, and the hand of 



the boatman the power. A door 

 moved on its hinges is another 

 example. A wheel-barrow is a 

 lever of the second class, the ful- 

 crum being the point at which the 

 wheel presses upon the ground, 

 the barrow and its load the weight, 

 and the hands the power. Nut- 

 crackers are two levers of the second class, the hinge which unites them being 

 the fulcrum, the resistance of the shell placed between them the weight, and 

 the hand the power. 



What are ex- ^03. A pair of sugar-tongs rep- p^j, 64^ 



amples of le- resents a lever of the third class, 

 third cufsa ?^* ^ which the power is appUed be- 

 tween the fulcrum and the resist- 



ance, or weight. In Fig. 64, the fulcrum is at a, 

 the resistance is the piece of sugar to be lifted at 

 5, and the power is the fingers applied at c. 

 When a man raises a ladder against a wall, he 

 employs a lever of the third class ; the fulcrum 

 being the foot of the ladder resting upon the 

 ground, the power being the hands applied to 

 raise it, and the resistance being the weight of the ladder. 



206. In levers of the third class, the power, 

 being between the fulcrum and the weight, 

 will be at a less distance from the fulcrum than 

 the weight ; and, consequently, in this form 



of lever the power must be always greater than the 



weight. 



What is the re- 

 lation betireen 

 the power and 

 the weight in 

 levers of the 

 third class f 



Thus (in No. 3, Fig. 01), if the length from the point where the weight, "W, 

 is suspended to F bo three times the length of P F, then a weight of 100 

 pounds suspended at "W will require a power of 300 applied at P to sustain it. 



