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LI II. On the Motion of floating Bodies in running Water, 

 By Peter Barlow, Esq., of the Rcyal Military Aca- 

 demy, Woolwich, 



To Mr. Til loch , — Sir, 

 In your Magazine for March, I have observed a letter from 

 Mr. G. Or, wherein he endeavours to account for some par- 

 ticular circumstances in the motion of floating bodies in ' 

 running waters, that have been noticed by Capt. Burney, 

 and which, it seems, formed the subject of a paper that was 

 read before the Royal Society. In order to accomplish this, 

 Mr. Orr has called in the assistances of the inclined plane, 

 and the doctrine of gravity, or, as he calls them, the laws of 

 matter and motion. Mr. Orr will excuse me, at least I hope 

 so, when I inform him that he does not seem to compre- 

 hend what those laws are. He is not, perhaps, aware that, in 

 the sense that he attaches to them, he is reviving the old 

 exploded notions of Aristotle, which ever since the time of 

 Galileo have been known to be erroneous : — that celebrated 

 philosopher proved that, by the law of gravity, all bodies, 

 whatever their magnitude and density may be, fall through 

 equal spaces in equal times ; and when this is not the case, 

 it arises from those laws being counteracted by some other 

 force .Thus in the case given by Mr. Orr, of two globes of 

 equal magnitudes and different densities rolling down an in- 

 clined plane, were thev left solely to the action of gravity, — ■ 

 for instance, had the experiment been made in vacuo, they 

 would both have descended in the same time; and the reason 

 they do not in ordinary experiments, is because the force of 

 gravity is counteracted by the resistances of the atmosphere; 

 for, both bodies having equal dimensions, they experience 

 equal resistances in their descent, and consequently that 

 body which oppose? the greatest force to this resistance, 

 that is, the heaviest body, will descend with the greatest 

 velocity. We see, therefore, that instead of gravity being 

 use of the different rales of descent of the two bodies, 

 the circumstance must be attributed to its laws being coun- 

 teracted. Mr. Orr is not less mistaken, in considering this 

 as a parallel case to two bodies floating in running water; 



for 



