72V NATURAL PHILOSOPHY. 2) 



P : W : : Interval : Circumference. 

 100 Ibs. : x : : / 4 in. : 96 in. x 3.1416. 

 x = 120,637 -fibs. 



17. What is the object of the big balls cast on the ends of tht 

 handle of the screw used in copying-presses ? 



By their inertia and centrifugal force they make the motion 

 more uniform and continuous. 



1 8. In a steelyard 7. ft. long, the distance from the 'weight- 

 hook to the fulcrum-hook is i in. How heavy a body can be 

 weighed with a I Ib. weight at the further end ? 



84 in. 2 in. = 22 in. 1 Ib. x 22 = 22 Ibs. = P. 22 Ibs. +- 2 = 11 Ibs. = W. 



19. Describe the change from the \st to the ^d class of levers, 

 in the different ways 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 F 

 between, this is a lever of the ist class. When throwing 

 dirt, the left hand at one end of the spade is the F ; 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 F and the W, this is a 

 lever of the 3d class. 



20. Why are not blacksmiths' and fire tongs constructed on 

 the same principle ? 



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

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



21. In a lever of the $d class, what W will a P of 50 Ibs. 

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



P : W : : Wd : Pd . 



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

 x = 12V a Ibs. 



22. In a lever of the id class, what W will a P of 50 Ibs. 

 balance, with a lever 12 feet long and W ^ feet from the Ff 



50 lb. : x : : 3 ft. : 12 ft. 



x = 200 Ibs. 



