2o the SURFACES of BODIES. 31 



tuating weight is the meafure of the refiftance of the air on the 

 body and the arm, and of the fri6:ion of the axis. 



4. Next, to find what part of the retardation is owing to 

 the fridion on the axis, and the refiftance of the air againft the 

 arm, both the a(5luating weight and the refifting body were ta- 

 ken off, and, inftead of the latter, a very thin bit of lead of 

 the fame weight was put on ; then various fmaller acfluating 

 weights were employed, till at laft fuch a one was found as 

 gave to the machine the very fame degree of uniform motion 

 as it had before, when the refifting body and the larger aduating 

 weight were employed. Then, the degree of velocity being the 

 fame in both cafes, the fmaller acftuating weight will be the 

 meafure of the fridlion on the axis, and the refiftance of the 

 air to the arm ; both of which, however, were reduced to as- 

 fmall a quantity as poflible, the former by means of fridlion- 

 wheels, and the latter by being made thin and feather-edged. 



5.^ Subtracting. now the lefs weight from the greater, the 

 remainder is the meafure of the refiftance of the air againft the 

 body alone ; that is, when reduced for the different lengths of 

 lever, namely, by diminilhing the remaining weight in the ra- 

 tio of the length of the arm to the radius of the axis, meafured 

 to the middle of the thread. Thus, then, we obtain a weight 

 which is the meafiire of the refiftance of the air againft a given 

 furface, moving with a given velocity ; that is, a weight which 

 is equal to the preffure of the air againft the furface, or which, 

 if it were laid upon and uniformly diffufed over the furface 

 when it is a plane, would prefs the furface juft as much as 

 the air does. 



6. Then, laftly, finding what altitude a column of air muft 

 have, which is of tlie weight of the aforefaid remaining weight 

 reduced, and whofe bafe is the plane of the refifting furface ; 

 it will be the altitude of the column of the fluid whofe weight 

 or preffure is equal to the refiftance, and which preffure would 

 confequently generate the fame velocity in the fluid. 



7. The 



