178 LECTURE XV. 



In cases where a motion not quite equable is required, as it sometimes hap- 

 pens in the construction of clocks, but more frequently in orreries, the wheels 

 may either be divided a little unequally, or the axis may be placed 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 be 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, tlie 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 distance of the centres of 

 the two wheels: consequently, when the wheels are equal, the central wheel 

 makes two revolutions, every time that the exterior wheel travels round it. 

 If the central wheel be fixed, and the exterior 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 ma- 

 chine; the wheel is fixed on an axis, which also carries a plate furnished 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 application 



