igo 



GENERAL SCIENCE 



Lifting jacks, cotton and hay presses, letter presses, 

 vises, the screw propeller of ships, and electric fans are 



JACK-SCREW 



COPYING PRESS 



familiar examples of the practical uses to which the 

 screw is put. It is also commonly used in machinery 



and woodworking. 

 The micrometer screw 

 can be used to measure 

 the thickness of a hair. 

 The speed counter is a 

 screw turning in a 



A SCREW FOR DETERMINING THE SPEED 

 OF ROTATING WHEELS 



notched wheel and is 

 used to determine the 

 number of revolutions per second made by a wheel. 



Problems. Find the mechanical advantage of an inclined plane 

 12 feet long and 3 feet high. Find the force required to roll a 

 600 pound barrel up the plane. How much work will be done in 

 rolling the barrel to the top of the plane? 



127. Pulleys. The pulley is a modified first or second 

 class lever. The center of the pulley is the fulcrum, 

 the force can be thought of as though it were applied at 

 one end of the diameter of the pulley. The weight is at 

 the other end of the diameter, when the pulley is a first- 



