32 On the RESISTANCE of the AIR 



7. The body ufed to affix to the end of the arm, in the fol- 

 lowing abftract, was a hemifphere of pafteboard, the hollow 

 part being covered with a flat circle of the fame, that either 

 the round or the flat fide might be made to go foremoft againft 

 the air. The diameter of the hemifphere was 6| inches ; and 

 confequently the area of its great circle, or flat fide, was 32 

 fquare inches or | of a fquare foot, and it weighed 4 oz. 3 dr. 

 avoirdupois. The hemifphere being fixed on the end of the 

 arm, with either fide foremoft, by a medium of feveral times, 

 and different ways of meafuring, it was found, that the radius 

 of the axis, including half the thicknefs of the thread, was 

 1.043 inches, and the length of the arm, meafured to the centre 

 of the hemifphere, was 53.34 inches ; fo that the two radii, 

 namely of the path of the body and of the axis, are to each 

 other as 53.34 to 1.043, or as 51. 14 to 1 : And therefore every 

 experimented actuating weight mull be divided by 5 1 . 14 or 5 1 \ , 

 to reduce it to the equivalent weight acting at the centre of the 

 hemifphere. 



8. The times of revolutions of the arm were counted by a 

 peculiar pendulum clock, beating feconds, which was made for 

 the purpofe. The method was thus : The clock being placed 

 clofe by the machine, and the hemifphere and actuating weight 

 fixed in their places, an afliftant held the hemifphere in a parti- 

 cular fituation by his hand, while a fecond afliftant audibly 

 counted the beats of the clock, beginning at 50 feconds, and 

 counting on from 1 to 10, which confequently would end at 

 60 or o ; and the inftant he pronounced 10, the firft afliftant 

 let the hemifphere go. The confequence was, it began at firft 

 to move very flowly, and gradually increafe for a fhort time, 

 and then to move uniformly. The firft afliftant, keeping his 

 ftation, called out at every time the body pafled him, in its re- 

 volution, and the other afliftant called out the correfponding 

 number of feconds and half feconds beat by the clock, which 

 I inftantly wrote down with a pencil on a paper held in my 



hand, 



