500 



GENERAL SCIENCE 



by a man without the aid of a hammer. Machines which save 

 force are said to have a mechanical advantage of force. 



Some machines have a mechanical advantage of speed. Many 

 of the common machines around our homes, such as the mowing 

 machine, the egg beater, the sewing machine, etc., have a mechanical 

 advantage of speed. 



Bobbin Winder Adjusting Arm 



Bobbin Winder Bracket (Belt Co 

 I 



Bobbin Window Frame 



Bobbin Winder 

 Spindle Worm 

 Bobbin Winder 



Friction 

 Ring (Rubber) 

 Bobbin Winder 



Pulley 

 Balance Wheel 



Bobbin Winder 



Thread Guide 



Cam 



Winder 

 Worm Wheel 



\ S Bobbin Winder Stop Latch 

 Feed Regulator Thumb Screw 

 Crank Connecting Rod 



Tension Disc I 

 Thread Take-up Spring. 

 Prener Foot Thumb Screw 



Hook Shaft Crank 



Rotating Hook Bobbin C 



FIG. 383. The Sewing Machine. 



Still other machines, such as the typewriter, the handle of a 

 pump, a pulley, have a mechanical advantage of position or direction. 

 Machines may have several mechanical advantages, but it is impos- 

 sible for machines to have a mechanical advantage of speed and 

 mechanical advantage of force at the same time. 



The Amount of Work a Horse Does. A horse walking at a rate 

 of 2J miles per hour should not exert a force of more than ^ to J 

 of its own weight, if the horse is doing continuous work all day. 



