WHEEL AND AXLE. EXAMPLES. 57 



therefore, the force must be considered as acting at the 

 centre of the rope ; hence the diameter of the rope must 

 be added to the diameter of the wheel. 



There are various forms of the wheel and axle. In the 

 common windlass, motion is given to the axle by means of 

 a winch, which is a lever like the handle of a grindstone. 

 The windlass used in digging wells has usually four pro- 

 jecting levers or arms. The wheel used in steering a ves- 

 sel is furnished with pins in the circumference, to which 

 the hand is applied in turning it. In the capstan (for 

 weighing anchor) the axis is vertical, and horizontal levers 

 are applied around it, so that several men may work at 

 once. The power of all these forms is easily calculated 

 by the rule of virtual velocities that is, that the velocity 

 with which the power moves is as many times greater 

 than the velocity of tlic Aveight, as the weight exceeds the 

 power. A simple and convenient rule for computing in 

 numbers the power of wheel-work is the following : Multi- 

 ply all the numbers together which express either the cir- 

 cumferences or diameters of the large wheels, and then 

 multiply together all the numbers which express the diam- 

 eters df the smaller wheels or pinions; divide the greater 

 number by the less, and the quotient will be the power 

 sought. 



BAND AND COG WHEELS 



Where great power is required, several wheels and 

 axles may be combined in a man- Fig. 59. 



ner corresponding with that of the 

 compound system of levers already 

 explained. In this case the axis of 

 one wheel acts on the circumference 

 of the next, producing a continued 

 slower motion, and increasing the 

 power in a corresponding degree. Combined cog-ivheds. 

 The wheels are made thus to act by means of cogs or teeth, 

 3* 



