1 , ON MACHINERY. l/y 



of a given force, or to reserve a part of it for future use. Tiiis is generally 

 effected by suffering a weight to descend, which has been previously raised, 

 or a spring to unbend itself from a state of forcible flexure, as is exemplified 

 in the weights and springs of clocks and watches. The common kitchen 

 jack is also employed for protracting and equalising-the operation of a weight: 

 in the patent jack the same effect is produced by an alternate motion, the 

 axis being impelled backwards and forwards, as in clocks and watches, by 

 means of an escapement, and the place of a balance spring being supplied by 

 the twisting and untwisting of a cord. 



In these machines, as well as in many others of greater magnitude, the fly 

 wheel is a very important part, its velocity being increased by the operation 

 of any part of the force which happens to be superfluous, and its rotatory 

 power serving to'continue the motion when the force is diminished or with- 

 drawn. Thus, when a man turns a winch, he can exert twice as nxiich force 

 in some positions as in others, and a fly enables him in some cases to do 

 nearly one third more work. In the pile engine, also, without the help of 

 the fly, the horses would fall for want of a counterpoise, as soon as the 

 weight is disengaged. Such a fly ought to be heavy, and its motion must 

 not be too rapid, otherwise the resistance of the air will destroy too much of 

 the motion ; but in the kitchen jack, as well as in the striking part of a clock, 

 where the superfluous force is purposely destroyed, the fly is made light, and 

 strikes the air with a broad surface. An effect similar to that of a fly and a 

 spring is sometimes produced in hydraulic machines by the introduction of an 

 air vessel, the air contained in which is compressed more or less according to- 

 the intensity of the force, and exerts a more uniform pressure in expelling the 

 fluid which is forced irregularly into it. 



