78 



THE ART OF PROJECTING, 



wooden pulley s^ about an inch in diameter, is made to 

 turn upon an axis that is made fast in the post/, and 

 with such adjustment that the pulley rests upon the 



Fig, 59, 



plate a and turns by friction on that plate. It is best 

 to have a thin india rubber ring upon the friction pulley 

 to insure it from slipping. Above the pulley the mirror 

 m is so mounted as to swing in azimuth, and is made 



to do this by a wire fastened to it 

 at its hinge, and bent into a loop / 

 at its lower end, which is opposite 

 the face of the pulley s. Another 

 twist in the wire at o will be needed, 

 for a pin which is fast in the post 

 / ; this will make a lever of the 

 wire /, with the fulcrum at ^, and if 

 it is properly fastened to the hinge 

 Fig. 60, Qf |-i^g mirror will cause it to vibrate 



in a horizontal plane when the plate a revolves. 



