'2QS PHILOSOPHICAL TRANSACTIONS. [ANNO I 78'2. 



city or cohesion. In the former kind we may rank the harder kinds of cast iron, 

 and in the latter, soft tempered clay. 



While the philosophical world was divided by the dispute about the old and 

 new opinion, as it was called, concerning the powers of bodies in motion, in 

 proportion to their different velocities: those who held the old opinion contend- 

 ing, that it was as the velocity simply, asked those of the new, How, on their 

 principles, they would get rid of the conclusions arising from the doctrine of un- 

 elastic perfectly hard bodies? They replied, they found no such bodies in nature, 

 and therefore did not concern themselves about them. On the other hand, those 

 of the new opinion asked those of the old, How they would account for the case 

 of non-elastic soft bodies, where, according to them, the whole motion lost by 

 the striking body was retained in the two after the stroke, the two bodies moving 

 together with the half velocity, though the two non-elastic bodies had been 

 bruised and changed their figure by the stroke; for, if no motion was lost, the 

 change of figure must be an effect without a cause? To obviate this, those of the 

 old opinion seriously set about proving, that the bodies might change their figure, 

 without any loss of motion in either of the striking bodies. 



Neither of these answers have appeared to me satisfactory, especially since my 

 mill experiments; for with respect to the first, it is no proper argument to urge 

 the impossibility of finding the proper material for an experiment, in answer to 

 a conclusion drawn from an abstract idea. On the other hand, if it can be 

 shown, that the figure of a body can be changed, without a power, then, by 

 the same law, we might be able to make a forge hammer work on a mass of soft 

 iron, without any other power than that necessary to overcome the friction, re 

 sistance, and original vis inertias, of the parts of the machine to be put in mo- 

 tion; for, as no progressive motion is given to the mass of iron by the hammer, 

 it being supported by the anvil, no power can be expended that way; and if none 

 is lost to the hammer from changing the figure of the iron, which is the only 

 effect produced, then the whole power must reside in the hammer, and it would 

 jump back again to the place from which it fell, just in the same manner as if it 

 fell on a body perfectly elastic, on which, if it did fall, the case would really 

 happen: the power therefore to work the hammer would be the same, whether 

 it fell on an elastic or non-elastic body ; an idea so very contrary to all experience, 

 and even apprehension, of both the philosopher and vulgar artist, that I shall 

 here leave it to its own condemnation. 



As nothing however is so convincing to the mind as experiments obvious to 

 the senses, I was very desirous of contriving an experiment in point; and as I 

 saw no hopes of finding matter to make a direct experiment, I turned my mind 

 towards an indirect one; so circumscribed however, as to prove incontestably, 

 that the result of the stroke of two non-elastic perfectly hard bodies could not 



