36 MECHANIC POWERS. 



equal parts as it will contain, each equal to G D, 

 the single weight P (which we may suppose to be 

 one pound) will serve for weighing any thing as 

 heavy as itself, or as many times heavier as there are 

 divisions in the arm D X, or any quantity between 

 its own weight and that quantity. As for ex- 

 ample, if P be one pound, and placed at the first 

 division 1 in the arm D X, it will balance one 

 pound in the scale at W ; if it be removed to the 

 second division at 2, it will balance two pounds in 

 the scale ; if to the third, three pounds ; and so 

 on to the end of the arm D X. If any of these 

 integral divisions be subdivided into as many equal 

 parts as a pound contains ounces, and the weight 

 P be placed at any of these subdivisions, so as to 

 counterpoise what is in the scale, the pounds and 

 odd ounces therein will by that means be ascer- 

 tained. 



THE WHEEL AND AXIS. 



This mechanic power consists of a wheel with 

 an axis or axle fixed to it, so as to turn round with 

 it ; the power being applied at the circumference 

 of the wheel, the weight to be raised is fastened to 

 a rope which coils round the axle. 



A B (Plate 2. fig. 1.) is a wheel, and C D an 

 axis fixed to it, and which moves round with it. 

 If the rope which goes round the wheel be pulled, 

 and the wheel turned once round, it is evident that 

 as much rope will be drawn off as the circumfer- 

 ence of the wheel ; but while the wheel turns 

 once round, the axis turns once round ; and con- 

 sequently the rope by which the weight is sus- 

 pended will wind once round the axis, and the 

 weight will be raised through a space equal to the 

 circumference of the axis. 



