22 



MECHANICS. 



Fig. 1. 



with the air-pump. Two fan-wheels, made of sheet 

 tm, one (a) striking the air with 

 its edges, and the other (6) with 

 its broad faces {Fig', l), are set in 

 motion ahke ; b is soon brought to 

 rest, wliile a continues revolving a 

 long time. If now they are placed 

 under the receiver of an au*-pump, 

 the air exliausted, and motion given 

 to them alike by the rack-work d, 



Fans ruolnns^m,, vacuum. l]^Qy will both COUtinue iu motioU 



during the same period. 



There is no machmery made by man free from the 

 checking influence of friction and the air ; and for tliis 

 reason, no artificial means have ever devised a perpet- 

 ual motion by mechanical force. But we are not with- 

 out a proof that motion will continue without ceasing 

 when nothing operates against it. The revolutions of 

 the planets in their orbits furnish a sublime instance ; 

 where removed from all obstructions, these vast globes 

 wheel round in their immense orbits, through success- 

 ive centuries, and with unerrmg regularity, preserving 

 undiminished the mighty force given them when first 

 launched into the regions of space. 



To set any body in motion, a force is requisite, and 

 the heavier the body, the greater must be the force. A 

 small stone is more easily thrown by the hand than a 

 cannon ball ; speed is much more easily given to a skiff 

 than to a large and heavy vessel ; but the same force 

 which sets a body in motion is required to stop it. 

 Thus, a wheel or a grindstone, made to revolve rapidly, 

 would require as great an effort of the arm to stop it 



