MECHANIC TOWERS. 39 



circumference of the second wheel exceeds that of 

 the first axis. 



In order to a balance here, the power must be to 

 the weight, as the product of the circumferences, 

 or diameters of the two axes multiplied together, 

 is to the circumferences or diameters of the two 

 wheels. 



This will become sufficiently clear, if it be con- 

 sidered as a compound lever, which was explained 

 above. Instead of a combination of two wheels, 

 three or four wheels may work in each other, or 

 any number ; and by thus increasing the number 

 of wheels, or by proportioning the wheels to the 

 axes, any degree of power may acquired. 



To this sort of engine belong all cranes for rais- 

 ing great weights; and in this case, the wheel may 

 have cogs all round it instead of handles ; and a 

 small lanthorn, or trundle, may be made to work 

 in the cogs, and be turned by a winch ; which 

 will make the power of the engine to exceed the 

 power of the man who works it, as much as the 

 number of revolutions of the winch exceeds those 

 of the axle C D (Plate 2. fig. 1). when multiplied 

 by the excess of the length of the winch above the 

 length of the semi-diameter of the axle, added to 

 the semi-diameter or half-thickness of the rope K 

 by which the weight is drawn up. Thus, suppose 

 the diameter of the rope and axis taken together 

 to be 1.3 inches, and consequently half their dia- 

 meter to be 6J- inches, so that the weight W will 

 hang at 6J inches perpendicular distance from 

 below the centre of the axle : now, let us suppose 

 the wheel A B, which is fixed on the axle, to 

 have 80 cogs, and to be turned by means of a 

 winch 6J inches long, fixed on the axle of a 

 trundle of eight staves, or rounds, working in the 



d 4 



