Wheel and Axle. 109 



gravity of the parts of the system which the power has to 

 sustain, allowance is made for it, in the case of an equilibri- 

 um by adding it to the weight, when its action coincides with Fig. 

 that of the weight. But if, as in figure 93, the gravity of the 

 system CF is not exerted in the same direction with the power 

 q. BC, instead of being in the same direction with the power </, 

 is in the direction of the resultant of the gravity of the system, 

 and the force exerted independently of gravity. 



Of the Wheel and Axle. 



185. The wheel and axle consists in general of a grooved 

 wheel, and a cylinder passing perpendicularly through the cen-Fig. 96. 

 tre of the wheel, and resting at its extremities upon two fixed 

 supports F, F. A power q, applied in the direction of a tangent 



to the circumference of the wheel, turns this wheel, together with 

 the cylinder, which being firmly fixed to it, takes up successive- 

 ly the different parts of the cord Dp and with it the weighty, 

 which it is proposed to elevate or draw toward the cylinder. 



Sometimes instead of a wheel, bars E* E* in the form of Fig. 96, 



98 99 



radii are the points at which the power is applied, and by which 

 the same effect is produced. At other times the extremities of 

 the cylinder are provided with winches <?, </, at which the mov- Fi 

 ing force is exerted. 



When the axis of the cylinder is vertical, the machine is F| 99 

 called a capstan. It is in this position that it is used on board of 100. 

 vessels, with this difference in the construction, however, that the 

 figure of the axis is made conical instead of being cylindrical, 

 that it may be worked more easily when the rope, having reach- 

 ed the lowest point, in turning to retrace its course would tend to 

 check the motion. 



186. But however the machine is placed, it will be seen that 

 the action of the power and that of the weight which it is pro- 

 posed to raise, are not exerted in the same plane, but in planes 

 that are parallel or nearly so. The power produces two effects, 

 one of which is exerted against the weight, and the other against 

 the supports. In case of an equilibrium, these effects may be 

 determined in the following manner. 



