17G BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



Desaguliers shows, in his fifth lecture, that this difference 

 arose principally from the resistance of the air. To remedy 

 the defects of these experiments, Mr. Atwood contrived his 

 apparatus. 



Of the mass moved. In order to obtain an adequate idea 

 of the laws that are observed in the communication of mo- 

 tion, and observe the effects of the moving force, the inter- 

 ference of all other force should be prevented. The bodies 

 impelled should be conceived to exist in free space, and be 

 void of gravity or weight ; so that to a given substance va- 

 rious degrees of force may be applied. This indeed cannot 

 be effected in bodies falling freely near the earth's surface ; 

 we cannot abstract the natural gravity or weight from any 

 substance whatsoever ; for the same substance is always im- 

 pelled by the same force of gravity, which admits not of 

 increase or diminution. 



Yet, this difficulty may be obviated by balancing two 

 equal weights, joined by a flexible line, which goes over a 

 pulley. The axle of the pulley must rest on wheels con- 

 structed for the purpose of diminishing friction. 



The motive force of gravity being destroyed by the con- 

 trary and equal action of the weights, they will remain 

 quiescent till some force is applied to them. When any im- 

 pulse is communicated to them in a vertical direction, they 

 will afterwards be observed to describe equal spaces in 

 equal times, or will move uniformly ; and the velocity com- 

 municated will be precisely the same, as if the same impulse 

 had been impressed on a quantity of matter equal to the 

 two bodies existing in free space without gravity, due allow- 

 ance being made for the inertia of" the wheels. 



Thus, in this instrument (Fig. 1*76, page 175), which is 

 constructed to illustrate this subject experimentally, there 

 are two equal weights, A, B, affixed to the extremity of a 

 very fine and flexible silk line. This line is stretched over a 

 brass wheel, abed, movable round a horizontal axis. The 

 two weights, A, B, being precisely equal, and acting against 

 each other, when the least weight is super-added to either, 

 it will preponderate. 



When the weights, A, B, are set in motion by the action 

 of any weight, which Mr. Atwood called m, the sum of A 

 added to B, added to m, would constitute the whole mass 

 moved ; but then there is to be added the inertia of the 

 materials which must necessarily be used in the communica- 



