26 ANSWERS TO PRACTICAL QUESTIONS 



17. Describe the change from the 1st to the 3d class of 

 levers, in the different ivays of using a spade. 



When digging, the ground at the back of the spade is the 

 F ; the ground lifted is the W ; and the hand at the other end 

 is the P. As the "W" is at one end, P at the other, and the P 

 between, this is a lever of the 1st class. When throwing dirt, 

 the left hand at one end of the spade is the P ; the dirt at the 

 other end is the W, and the right hand between the two is the 

 P. As the P is between the P and the W, this is a lever of 

 the 3d class. 



18. Wliy are not blacksmiths' tongs and fire tongs con- 

 structed on the same principle ? 



The former are of the 1st class, as power is required : the 

 latter of the 3d class, as rapidity only is necessary. 



19. In a lever of the 3d class, what W will a P of 5O 

 Ibs. balance, if one arm is 12ft. and the other 3 ft. long? 



50 Ibs. : x : : 12 ft. : 3 ft. 

 x = 12|- Ibs. 



96 2O. In a lever of the 2d class, ivhat W will a P of 

 50 Ibs. balance, with a lever 12 feet long and W 3 feet 

 from the F? 



50 Ibs. : x:: 3ft. : 12ft. 

 x = 200 Ibs. 



21. In a lever of the 1st class, what W will a F of 5O 

 Ibs. balance, with a lever 12 ft. long and the F 3 ft. from 

 the W? 



50 Ibs. : x : : 3 ft. : 9 ft. 

 x = 150 Ibs. 



22. In a wheel and axle, the F = 4O Ibs., W = 36O Ibs., 

 diameter of aocle = # in. Required the circumference of 

 the wheel. 



P : W : : diameter of axle : diam. of wheel. 



40 Ibs. : 360 Ibs. : : 8 in. : x = 72 in. = 6 ft., the diameter of wheel. 

 6 ft. x 3.1416 = 18.85 ft., the circumference of the wheel. 



