30 O/i the RESISTANCE of the AIR 



the accuracy of the branch, for the improvement of which I 

 am more particularly folicitous at this time ; and therefore the 

 laws here deduced are not meant to be extended to other fluids 

 of a different nature. 



2. The machine with which thefe experiments were per- 

 formed, was made after the pattern of, and by the fame work- 

 man as that which is defcribed by the late excellent Mr Ro- 

 bins, in the firft volume of his works, as publifhed by Dr 

 Wilson, and of which a view is inferted in that volume, at 

 leaft of the principal parts of it. SuflBce it, therefore, in this 

 place, juft to obferve, that it confifts of a fmall vertical axis, 

 with a long horizontal arm connedted with it. A body of any 

 form is fixed on the extremity of the arm ; then a fine, but 

 ftrong filken thread, or cord, is wound about the axis, with a 

 given fmall weight at the end, which is paffed over a vertical 

 puUy, and left to defcend by its weight, and fo turning the axis, 

 gives motion to the arm and body at the end of it. Hence it is 

 evident, that a flow motion of the axis, or of the adluating 

 weight, will give a very quick motion to the refifl;ing body at the 

 end of the arm ; this latter being to the former indeed, as the 

 length of the arm, meafured to the centre of the body, is to 

 the radius of the axis, which, in thefe experiments, was as 

 jiy to I. 



3. The adluating weight would defcend continually with an 

 accelerated velocity, were it not for the friiflion of the axis, and 

 the refiftance of the air to the arm and the body placed at the 

 end of it. But this refiftance always increafing with the velo- 

 city, and indeed as the fquare of it, it muft needs happen, 

 that, by the refiftance conftantly gaining on the velocity, this 

 will foon arrive at its maximum, and after that proceed with 

 a uniform motion, the refiftance neither gaining on the ve- 

 locity, nor the velocity on the refiftance, but each mutyally 

 balancing the other. As foon as this happens, then the ac- 

 tuating 



