138 PNEUMATICS. 



weights, independent of each other, and which turn 

 equallyfree on their axles in the frame. Each mill has 

 four thin arms or sails fixed into the axis: those of 

 the mill a have their planes at right angles to its 

 axis, and those of b have their planes parallel to it. 

 Therefore, as the mill a turns round in common 

 air, it suffers little resistance, because its sails cut 

 the air with their thin edges: but the mill /; suffers 

 much more, because the broad side of its saiis 

 move against the air when it turns round. In 

 each axle is a tine pin near the middle of the frame, 

 which goes quita through the axle, and stands out 

 a little on each side of it: under these pins a slider 

 may be made to bear, which will hinder the mills 

 from going when a strong spring is set or bent 

 against the opposite ends of the pins. 



Having set this machine upon the pump-plate, 

 draw up the slider to the pins on one side, and set 

 the spring at bend on the opposite ends of the pins: 

 then push down the slider,' and the spring acting 

 equally strong upon each mill, will set them both 

 going with equal forces and velocities; but the 

 mill a will run much longer than the mill b, be- 

 cause the air makes much less resistance against 

 the edges of its sails than against the sides of the 

 sails of b. 



Draw up the slider again, and set the spring upon 

 the pins as before; then cover the machine with 

 the receiver upon the pump-plate; and having ex- 

 hausted the receiver of air, push down the wire 

 through the collar of leathers in the neck upon 

 the slider; which will disengage it from the pins, 

 and allow the mills to turn round by the impulse 

 of the spring: as there is no air in the receiver to 

 make any sensible resistance against them, they 

 will both move a considerable time longer than 



