VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. 2Q7 



with which they have abundant concern, to wit, the non-elastic soft bodies, of 

 which water is one, which they have much to do with in their daily practice. 



Previous to the trying my experiment on mills I never had doubted the truth 

 of the doctrine, that the same velocity resulted from the stroke of both sorts of 

 non-elastic bodies; but the trial of those experiments made me clearly see at least 

 theinconclusiveness, if not the falsity, of that doctrine: because I found a result 

 which I did not expect to have arisen from either sort; and from which, when it 

 appeared from experiment, I could see a substantial reason why it should take 

 place in one sort, and that it was impossible that it could take place in the other; 

 for if it did, the bodies could not have been perfectly hard, which would be con- 

 trary to the hypothesis. Of this deduction I have given notice in my said tract 

 on mills, published in the Philos. Trans, for 175Q,* [Abridg. vol. ii. p. 338]. 



It may also be said, that since we have no bodies perfectly elastic, or perfectly 

 unelastic and soft, why should we expect any bodies perfectly unelastic and hard? 

 Why may not the effects be such as should result from a supposition of their 

 being imperfectly elastic joined with their being imperfectly hard? But here I 

 must observe, that the supposition appears to be a contradiction in terms. We 

 have bodies which are so nearly perfectly elastic, that the laws may be very well 

 deduced and confirmed by them; and the same obtains with respect to non- 

 elastic soft bodies; but concerning bodies of a mixed nature, which are by far 

 the greatest number, so far as they are wanting in elasticity, they are soft, and 

 bruise, yield, or leave a mark in collision ; and so far as they are not perfectly 

 soft they are elastic, and observe a mixture of the law relative to each; but im- 

 perfectly elastic bodies, imperfectly hard, come in reality under the same descrip- 

 tion as the former mixed bodies: for so far as they are imperfectly hard they are 

 soft, and either bruise and yield, or leave a mark in the stroke; and so far as 

 they want perfect elasticity, they are non-elastic; that is to say, they are bodies 

 imperfectly elastic, and imperfectly soft; and in fact I have never yet seen any 

 bodies but what come under this description. It seems therefore, that respecting 

 the hardness of bodies, they differ in degrees of it, in proportion as they have 

 a greater degree of tenacity or cohesion; that is, are farther removed from per- 

 fect softness, at the same time that their elastic springs, so far as they reach, are 

 very stiff; and hence we may by the way conclude, that the same mechanic 

 power that is required to change the figure in a small degree of those bodies that 

 have the popular appellation of hard bodies, would change it in a great degree 

 in those bodies that approach towards softness, by having a small degree of tena- 



* " The effect therefore, of overshot wheels, under the same circumstance of quantity and fall, 

 is at a medium double to that of the undershot : and as a consequence thereof, that non -elastic bodies, 

 when acting by their impulse or collision, communicate only a part of their original power; the 

 other part being spent in changing their figure in consequence of the stroke."— Orig. 

 VOL. XV. Q Q 



