VOL. XXXII.] PHILOSOPHICAL TRANSACTIONS. 573 



resistance I have proposed, it is a sufficient confirmation of the truth of this 

 theory. 



Only here I ought to observe, that I have supposed the globes to be stopped 

 by the whole resistance of the substance they move against, though in strictness 

 they are stopped only by the excess of that resistance above the action of gra- 

 vity upon them. But I have neglected the consideration of the action of 

 gravity, that being but small in proportion to the resistance, as will appear 

 from the globes being much more speedily stopped by this resistance, than they 

 would be by the action of gravity, if its force were applied upwards ; for by 

 that force alone, the globes would not be stopped, till they had measured spaces 

 equal to the heights above the resisting substance from whence they fell; which 

 heights bear a great proportion to the depths the globes in this experiment are 

 immersed into the yielding substance, as I have found upon trial. 



Hence therefore we may see, that the very method of reasoning, which 

 being applied erroneously, is supposed to prove Mr. Leibnitz's sentiment con- 

 cerning the force of bodies in motion, will, when justly used, confirm the other 

 opinion in relation to that matter. 



Thus may this experiment be made use of to invalidate that very opinion it is 

 brought to support. But another use may likewise be made of it; for it will 

 serve to illustrate what Sir Isaac Newton has more than once hinted, that the 

 resistance of fluids, which arises from the tenacity of their parts, decreases in 

 a less proportion than the velocity of the resisted body decreases;* for as this 

 resistance bears a great analogy to the resistance of the yielding substances we 

 have been here treating of, so we have found that the resistance of these sub- 

 stances does not much depend on the velocity of the body, against which the 

 resistance is applied. 



Postscript.^ — About a week after I had sent you the letter, containing my 

 observations on Polenus's experiment, I had the good fortune to hear an excel- 

 lent and learned friend of yours, to whom you had been pleased to show my 

 letter, give a very curious and weighty argument to confirm Sir Isaac Newton's 

 sentiment in relation to the resistance of fluids, which I had deduced from the 

 above-mentioned experiment; and as this very much pleased rne, I shall here 

 endeavour to send you an account of it in the following manner: 



Suppose pieces of fine silk, or the like thin substance, extended in parallel 

 planes, and fixed at small distances from each other. Suppose then a globe to 



* Vid. Philos. Nat. Princip Math. prop. 32, lib. 2, in Schol. Opticks. qu. 28, p. 339, 340.— Orig, 

 f This postscript it seems was by Sir I. Newton himself^ as stated by Dr. James Wilson, in his 



Life of Dr. Pemberton, prefixed to an edition of our authcwr's Course of Chemistry, in 24 lectures, 



printed in 1771. 



