VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. SOI 



one-half of the mechanic power is destroyed; or rather, in this case, remains 

 locked up in the springs, capable of being re-exerted whenever they are set at 

 liberty, and of producing a fresh mechanic.d effect, equivalent to the motion or 

 mechanical power of the two non-elastic soft bodies after their collision. 



Hence we must infer, that the quantity of mechanical power expended in dis- 

 placing the parts of non-elastic soft bodies in collision, is exactly the same as that 

 expended in bending the springs of perfectly elastic bodies; but the difference in 

 the ultimate effect is, that in the non-elastic soft bodies, the power taken to dis- 

 place the parts will be totally lost and destroyed, as it would require an equal 

 mechanic power to be raised afresh, and exerted in a contrary direction to restore 

 the parts back again to their former places; whereas, in the case of the elastic 

 bodies, the operation of half the mechanic power is, as observed already, only 

 locked up and suspended, and capable of being re-exerted without a further 

 original accession. 



Those ideas arose from the result of the experiments tried on the machine 

 described in my said tract on Mechanic Power, and were also communicated to 

 my very worthy and ingenious friend Wm. Russel, Esq., f. r. s., at the same 

 time that I showed him those experiments in 1759; but the mode of putting this 

 matter to a full and fair mechanical trial has since occurred; and though some 

 rough trials, sufficient to show the effect, were made on it, prior to offering the 

 paper on mechanical power to the Society in 1776, yet the machine itself I had 

 not leisure to complete to my satisfaction till lately; which I mention, to apo- 

 logize for the length of time that these speculations have taken in bringing 

 forward. 



Description of the machine for collision. — Fig. 6, pi. 5, shows the front of the 

 machine as it appears at rest when fitted for use. a is the pedestal, and ab the 

 pillar, which supports the whole; c, d, are two compound bodies of about a 

 pound weight each, but as nearly equal in weight as may be. These bodies are 

 alike in construction, which will be more particularly explained by fig. 7. These 

 bodies are suspended by 2 white fir rods, of about half an inch diameter, ef and 

 gh, being about 4 feet long from the point of suspension to the centre of the 

 bodies; and their suspension is on the cross piece 11, which is mortoised through, 

 to let the rods pass with perfect freedom; and they hang on 2 small plates filed 

 to an edge on the under side, and pass through the upper part of the rods. 

 Their centres are at k and 1, and the edges being let into a little notch, on 

 each side of the mortoise, the rods are at liberty to vibrate freely on their res- 

 pective points, or rather edges, of suspension, and are determined to one plain 

 of vibration, mn is a fiat arch of white wood, which may be covered with 

 paper, that the marks on it may be the more conspicuous. The cross piece 11 is 

 made to project so far before the pillar, that the bodies in their vibrations may 



