. ON MACHINERY. 137 



a little out of the centre ; and these eccentric wheels may either act on 

 other eccentric wheels, or, if they are made as contrate wheels, upon a 

 lengthened pinion. (Plate XV. Fig. 195, 196.) 



An arrangement is sometimes made for separating wheels which are in- 

 tended to turn each other, and for replacing them at pleasure ; the wheels 

 are said to he thrown by these operations out of gear and into gear again. 



When a wheel revolves round another, and is so fixed as to remain 

 nearly in a parallel direction, and to cause the central wheel to turn round 

 its axis, the apparatus is called a sun and planet wheel. In this case, the 

 circumference of the central wheel moves as fast as that of the revolving- 

 wheel, each point of which describes a circle equal in diameter to the dis- 

 tance of the centres of the two wheels : consequently, when the wheels are 

 equal, the central wheel makes two revolutions, every time that the ex- 

 terior wheel travels round it. If the central wheel be fixed, and the ex- 

 terior wheel be caused to turn on its own centre during its revolution, by 

 the effect of the contact of the teeth, it will make in every revolution one 

 turn more with respect to the surrounding objects, than it would make, if 

 its centre were at rest, during one turn of the wheel which is fixed : and 

 this circumstance must be recollected when such wheels are employed in 

 planetariums. 



Wheels are usually made of wood, of iron, either cast or wrought, of 

 steel, or of brass. The teeth of wheels of metal are generally cut by means 

 of a machine ; the wheel is fixed on an axis, which also carries a plate fur- 

 nished with a variety of circles, divided into different numbers of equal 

 parts, marked by small excavations ; these are brought in succession 

 under the point of a spring which holds the axis firm, while the intervals 

 between the teeth are expeditiously cut out by a revolving saw of steel. 

 The teeth are afterwards finished by a file ; and a machine has also been 

 invented for holding and working the file. (Plate XV. Fig. 197.) 



It is frequently necessary in machinery to protract the time of ap- 

 plication of a given force, or to reserve a part of it for future use. This is 

 generally effected by suffering a weight to descend, which has been previ- 

 ously 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 com- 

 mon kitchen jack is also employed for protracting and equalising the ope- 

 ration 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 ope- 

 ration of any part of the force which happens to be superfluous, and its ro- 

 tatory power serving to continue the motion when the force is diminished 

 or withdrawn. Thus, when a man turns a winch, he can exert twice as 

 much force in some positions as in others, and a fly enables him in some 

 crises 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 



