MOTION, MECHANICS, ETC. 177 



tion of motion. These materials are: 1. The large wheel, 

 abed; 2. The four brass friction- wheels, on which the 

 axle of the wheel, abed, rests ; these wheels are used to 

 prevent the loss of motion, which would be occasioned by 

 the friction of the axle, if it revolved on an immovable sur- 

 face ; 3. The weight of the line ; but this is too inconsider- 

 able to have any sensible effect. 



Of the resistance from the inertia of the pulleys. If the 

 whole mass of the wheels were accumulated in the circum- 

 ference of the wheel, abed, its inertia would be truly 

 estimated by the quantity of matter moved. If their figures 

 were regular, and the density distributed uniformly in each, 

 mathematicians would furnish us with rules for finding a 

 weight, which, being accumulated uniformly in the circum- 

 ference, a b c d, would exert an inertia equal to that of the 

 wheels. But as the figures are wholly irregular, recourse 

 must be had to experiment for the discovery of such a 

 weight. 



For this purpose a weight of thirty grains was affixed to 

 a silk line which did not weigh one-quarter of a grain ; this 

 line being wound round the wheel, the weight of thirty 

 grains, by descending from rest, communicated motion to 

 the wheel, and, by many trials, Avas observed to describe a 

 space of thirty-eight and a half inches in three seconds. 

 From these data we find the mass equivalent to the inertia 

 to be two ounces and three quarters. This is a mass equi- 

 valent to the inertia of the wheel, abed, and the friction- 

 wheels together.* 



The resistance to motion, therefore, arising from the 

 wheel's incrtion will be the same as if it were absolutely re- 

 moved, and a mass of 2 were accumulated in the circum- 

 ference of the wheel, abed. 



This being premised, suspend the pieces, or brass boxes, 

 A, B, by a silk line passing over the wheel, abed, and 

 make them balance each other ; now, if I add any weight, 



* Mr. Atwood proves in his work, t that the following formula 

 will give the required mass .^=p x, w here p signifies the 



weight, 30 gr. ; t the time, 3 seconds; d the space described by a 

 body in a second, 16 feet 1 inch, or 193 inches; s the space described 

 by the body, 38.5 inches ; and x the inertia sought. 



That is in figures for the present case, 3 o M 9 M i 930 equal to 

 1323 grains, or 2 3-4 oz. 335 



