12 



MECHANICS. 



seconds would be 128 feet per second, 

 or 7680 feet per minute. 



Independently of these difficulties, the 

 resistance which the atmosphere would 

 offer to such rapid motions, would be 

 so considerable as to produce a great 

 discordance between the effects ob- 

 served, and the laws which have been 

 laid down on the supposition that all 

 resistances to the free descent of the 

 body are removed. 



(35.) Nevertheless, the truth of 

 these laws can be established by the 

 most rigorous experiments ; and al- 

 though the impediments to which we 

 have just alluded cannot be directly 

 removed, they may be evaded. It oc- 

 curred to Mr. George Attwood, that, 

 if a force of the same kind with the 

 force of gravity, but of a much less 

 intensity, could be obtained, the de- 

 scent of bodies, actuated by such a 

 force, while it would be regulated by the 

 same laws as the descent of heavy bodies 

 by the force of gravity, would be so 

 slow that the resistance of the. air 

 would produce no sensible effect, and 

 at the same time all the particulars of 

 space, time, and velocity might be de- 

 liberately observed, and accurately mea- 

 sured. To realize this conception, he 

 passed a fine silken thread over a groove 

 in the edge of the rim of a wheel which 

 turned freely on an horizontal axle, and 

 from the ends of the thread he sus- 

 pended equal weights. In this state, 

 the weights were necessarily in equili- 

 brium. To one of the weights he added 

 a small quantity, so as to give it a 

 slight preponderance. It consequently 

 commenced to descend, causing the 

 lighter weight to ascend. Setting aside 

 the effects of the friction of the wheel 

 on which the string connecting the 

 weights rested, the descent of the weight 

 was, in this case, one of uniform acce- 

 leration, similar exactly to the descent 

 of a heavy body, but differing in this, 

 that the acceleration might be rendered 

 as slow as might be thought necessary 

 for the purposes of convenient and ac- 

 curate observation, by diminishing to 

 any degree the preponderancy given to 

 the heavier weight. 



(36.) As we have stated that light 

 and heavy bodies are equally accele- 

 rated by gravity, it might be supposed, 

 that, since the equal weights first sus- 

 pended from the thread counterpoise 

 each other, the additional weight sus- 

 pended from one end should descend 

 with as great velocity as it would have 



by the immediate action of gravity. This, 

 in fact, would be the case were the force 

 which gravity exerts upon it wholly 

 spent in producing its descent ; but it 

 should not be forgotten that the ascent 

 of the weight at the opposite end of the 

 thread is to be accomplished ; and since 

 the original weight placed upon the de- 

 scending end is only sufficient to coun- 

 terpoise it, it can have no share in 

 raising it. Its elevation, therefore, is 

 entirely effected by the force which 

 gravity impresses on the additional 

 weight placed at the descending end of 

 the string ; and all the force thus spent 

 in drawing up the opposite weight is 

 necessarily subtracted from the force 

 with which the additional weight at the 

 descending end falls. The additional 

 weight has also to draw down the de- 

 scending weight, and to give it as much 

 moving force in its descent as it gives 

 to the ascending weight in its ascent. 

 Hence it follows, that the smaller this 

 additional weight is in comparison with 

 the equal weights originally suspended, 

 the slower will be the rate of its de- 

 scent. 



It still remained, however, to remove 

 the effects of the friction of the wheel, 

 on which the thread connecting the 

 weights turned. Mr. Attwood accom- 

 plished this, by an ingenious combina- 

 tion of wheels, called friction- wheels, by 

 which the axle of the wheel carrying 

 the thread, instead of turning in cylin- 

 drical holes, rested on the edges of other 

 wheels, by which means, the friction 

 against the inner surface of the holes in 

 which the axle turned, was entirely 

 avoided ; and, if all friction was not 

 removed, as far as it affected the motion 

 of the weights, it was so far diminished 

 as to produce no sensible effect upon 

 the motion of the weights in the expe- 

 riments for which the apparatus was 

 used. 



(37.) A representation of this beau- 

 tiful and useful contrivance is given 

 injig. .8, (and on an enlarged scale in 

 fig. 9,) b c d is the rim of the wheel 

 over which the thread sustaining the 

 weights passes. The ends of the axle 

 of this wheel rest upon the rims of 

 two pairs of wheels, as is represented in 

 the figure, and already described. The 

 stand carrying the apparatus is sup- 

 ported by a strong pillar, and immedi- 

 ately under this stand is placed an 

 upright shaft C D, divided to inches, 

 half inches, and tenths, for the purpose 

 of measuring the rate of descent A 



