MECHANICS. 



23 



(53.) There are various ways in which 

 the power is applied to the wheel ; and 

 the weight suspended by the rope which 

 we have used is only to be understood 

 as a general representation of the me- 

 chanical effect of the power, however it 

 be applied. 



Sometimes pins are placed at equal 

 distances round the circumference of 

 the wheel, by which the hand may be 

 applied as the power. (See fig. 33.) An 

 instance of this manner of applying the 

 power will be seen in the wheel used to 

 work the rudder of a ship. 



Sometimes there is no wheel, but a 

 number of long levers are inserted in 

 the axle, and diverge from it like spokes 

 from the box of a wheel, and the opera- 

 tor moves the axle by these levers. (See 



fig- 3 5.) 



Frequently, as in the windlass, there 

 is not either a wheel or diverging spokes, 

 but simply holes, in which the workman 

 inserts the end of a strong bar of con- 

 siderable length, by which, as a lever, he 

 turns the axle and raises the weight. 

 He removes the bar as he works, from 

 one hole to another, the weight being 

 sustained by the action of the ratchet- 

 wheel in the interim. 



The windlass is frequently wrought 

 by a winch, as represented in Jig. 34. 

 Fig. 34. 



The cylinder is fixed upon the axle A B, 

 and at right angles to this axle a lever 

 B O is fixed, and from O another arm 

 O D extends, to which the hands of the 

 operator are applied. By means of this 

 simple contrivance, the operator can 

 work upon the machine through the 

 entire of each revolution, although not 

 in every part with equal effect. In this 

 ease the ratchet-wheel is less necessary 

 than in any of the former. 



(54.) The axle is sometimes horizon- 

 tal and sometimes vertical. In the wind* 



la-ss it is horizontal, also in most kinds of 

 cranes. In the capstan (fig. 35 .) it is ver- 

 tical. The advantage of its vertical posi- 

 tion in the capstan is very evident. The 



Fig. 35. 



workmen insert levers in holes made to 

 receive them, and walk round the axle, 

 pushing the levers before them. In this 

 manner a great number of men may 

 vvork together, and there is no intermis- 

 sion of the power even when different 

 men succeed each other. 



(55.) When several forces act at the 

 same time at different parts of the cir- 

 cumference of the same wheel, and all 

 perpendicular to the radii, their com- 

 bined effect is to be estimated in the 

 same manner as if they all acted at the 

 same point. 



But if they act with different levers, 

 each power is to be multiplied by the 

 lever by which it acts, and the sum of 

 these products is to be taken as the 

 total effect. (10.) 



(56.) In estimating the effect of the 

 resistance or weight when the rope by 

 which it acts lias any considerable 

 thickness, half the thickness of the rope 

 is to be considered as a part of the ra- 

 dius of the cylinder, as the force is 

 considered to be transmitted through 

 the centre or axis of the rope. 



The same may be said of the power, 

 when it acts by a rope of any consi- 

 derable thickness. 



(57.) In order to determine the 

 strength necessary to be given to the 

 pivots which support the wheel and 

 axle, it is necessary to determine the 

 pressures to which they are liable. 



If the axle be horizontal, the weight of 

 the machine will press upon the pivots ; 

 and if its centre of gravity be equally 

 distant from the pivots, its pressure will 

 be equally distributed between them, 

 half the weight of the machine pressing 

 upon each pivot. If, however, the axis 

 be vertical, the whole weight will rest 

 on the point of the lower pivot, 



