IN POPULAR PHYSICS. 23 



The number of vibrations per hour varies inversely as the 

 time of vibration of the pendulum. For t in the formula, 

 therefore, we may substitute the reciprocal of the number of 

 vibrations per hour. 



1 . JL . . 1_ . 1_ 



3600 ' x" ^3^088 ' V32T253 

 x = 3,609.36 times. 



THE MECHANICAL POWERS. 



94 1. Describe the rudder of a boat as a lever. 



The water is the F, the boat the "W, and the hand the P. 

 As the W is between the F and the P, it is a lever of the 

 second class. By similar reasoning it is easy to analyze the 

 remaining cases, a door, a door-latch, etc. 



952. Show the change that occurs from the second to 

 the third class of levers, when you take hold of a ladder at 

 one end and raise it against a building. 



At first the ground is the F at one end, the hand the P 

 at the other, and the ladder the W hanging between ; hence this 

 is a lever of the second class. After a little, the F remaining 

 the same, the P is applied at one end, near the F, and the 

 ladder is the W hanging at the other ; hence this is now a 

 lever of the third class. 



3. IVJiy is a pinch from the tongs near the hinge more 

 severe than one near the end ? 



Because in the former case the tongs are a lever of the 

 first class in the latter, of the third. In the first class there 

 is a gain of power, in the third a loss. 



4. Two persons are carrying a weight of 25 O lbs. 9 

 hanging between them from a pole 1O feet in length* 

 Wliere should it be suspended so that one ivill lift only 5O 

 Ibs. ? 



One lifts 50 Ibs. ; the other 200 Ibs. The proportionate 

 length of the arms of the lever should be the same as the 

 proportionate weights i.e., 1 to 4. 10-5-5 = 2, the unit of 



