VOL. LXXV.] PHILOSOPHICAL TRANSACTIONS. Qjg 



proceeding to relate them, I beg leave to recommend to those who may after- 

 wards be induced to repeat them, the following cautions, which are extremely 

 necessary to be attended to. Great care must be taken that the two surfaces 

 have exactly the same degree of roughness ; in order to be certain of which, 

 such bodies must be chosen as have no knots in them, and whose grain is so 

 very regular that when the two surfaces are planed with a fine rough plane, their 

 roughness may be the same, which will not be the case if the body be knotty, 

 or the grain irregular, or if it happens not to run in the same direction on both 

 surfaces. When you cannot depend on the surfaces having the same degree of 

 roughness, the best way will be to paste some fine rough paper on each surface, 

 which perhaps will give a more equal degree of roughness than can be obtained 

 by any other method. Now as the proof which I have already given depends only on 

 the motion of the body on the same surface, it is not liable to any inaccuracy of 

 the kind which the preceding cautions have been given to avoid, nor indeed to 

 any other, and therefore it must be perfectly conclusive. In the following expe- 

 riments the cautions mentioned above were carefully attended to. 



Exper. 1 . A body was taken whose flat surface was to its edge as 22 : 9, and 

 with the same moving force the body described on its flat side 33^ inches in 2% 

 and on its edge 47 inches in the same time. 



Exper. 2. A 2d body was taken whose flat surface was to its edge as 32 : 3, 

 and with the same moving force it described on its flat side 32 inches in 2 s , 

 and on its edge it described 37-1- inches in the same time. 



Exper. 3. I took another body and covered one of its surfaces, whose length 

 was 9 inches, with a fine rough paper, and by applying a moving force, it 

 described 25 inches in 2 s ; I then took off some paper from the middle, leaving 

 only -§- of an inch at the two ends, and with the same moving force it described 

 40 inches in the same time. 



Exper. 4. Another body was taken which had one of its surfaces, whose 

 length was 9 inches, covered with a fine rough paper, and by applying a moving 

 force it described 42 inches in 2 s ; some of the paper was then taken off from 

 the middle, leaving only l-f- inches at the two ends, and with the same moving 

 force it described 54 inches in 2 s ; I then took off more paper, leaving only \ of 

 an inch at the two ends, and the body then described, by the same moving force, 

 60 inches in the same time. In the last 2 experiments the paper which was 

 taken off the surface was laid on the body, that its weight might not be altered. 



Exper. 5. A body was taken whose flat surface was to its edge as 30 : 17 ; the 

 flat side was laid on the horizontal plane, a moving force was applied, and the 

 stage was fixed in order to stop the moving force, in consequence of which the 

 body would then go on with the velocity acquired till the friction had destroyed 

 all its motion ; when it appeared from a mean of 1 2 trials that the body moved, 



4 p 2 



