78 OUTLINES OF NATURAL PHILOSOPHY. 



It is supposed here, that the power and the weight 

 both act at right angles to the radius. 



a. The wheel and axle is nothing else but a lever, so 

 contrived as to have a continued motion about its 

 fulcrum. The principle of the virtual velocities is 

 here obviously applicable. If the power act not at 

 the circumference of the wheel, but at the extre- 

 mity of a handspike inserted in the wheel, the dis- 

 tance of that extremity from the centre of the axis, is 

 to be accounted the radius of the wheel. 



b. The Capstan, the Windlass, and other contrivances 

 of a similar nature, are nothing else than the wheel 

 and axle adapted to particular circumstances. 



c. The mechanical contrivance called a Crank, is a spe- 

 cies of wheel and axle. If the force which acts up- 

 on a crank urges it directly down and up, alternate- 

 ly, the effect is to that which would be produced if 

 the force acted at right angles to the arm of the 

 crank all round, as twice the diameter of a circle to 

 its circumference, or as seven to eleven nearly. 



d. The combinations of wheels, so useful in mecha- 

 nics, are generally reducible to the wheel and axle, 

 though the wheel which turns the other is not al- 

 ways upon the same axis with it. The motion, in 

 such cases, is communicated from the one wheel to 

 the other, either by belts or straps passing over the 

 circumferences of both, or by teeth cut in those cir- 

 cumferences, and working in one another. 



136. When 



