WHEEL AND AXLE. 77 



the wheel and of the axle, according to the rule of vir- 

 tual velocities already explained. 



The thickness of the rope has not been taken into 

 consideration. This is very small when compared with 

 the diameter of the outer wheel, but often considerable 

 when compared with that of the inner. To be strictly 

 accurate, therefore, the force must be considered as act- 

 ing 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 projecting levers or arms. The wheel 

 used in steering a vessel is furnished with pins in the 

 circumference, to which the hand is applied m 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 vir- 

 tual velocities — that is, that the velocity with which 

 the power moves is as many times greater than the ve- 

 locity of the weight, as the weight exceeds the power. 

 A simple and convenient rule for computing in num- 

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

 ply all the numbers together which express either the 

 circumferences or diameters of the large wheels, and 

 then multiply together all the numbers which express 

 the diameters of the smaller wheels or pinions ; divide 

 the greater number by the lesser, and the quotient will 

 be the power sought. 



