Haughton and Eeynolds — "Drag" of Air u^wn Air. 277 



XLVI. — Experiments made to deteemine the " Deag"^ of Aie upon" 

 AiE AND OF Watee tTPON Watee AT Low VELOCITIES. By the Rev. 

 Samuel Hattghton, M.D., D.C.L., and J. Emeeson Reynolds, 

 M.D. 



[Read, February 23, 1880.] 



A SPHEEicAi, ball o£ granite, unpolished, was suspended by a pianoforte 

 wire, and allowed to hang freely ; from the brass collar by which the 

 ball was suspended an index projected on each side, the pointed ends 

 of the indices traversing a graduated horizontal circle, whose centre 

 corresponded with the line of suspension. The suspended ball was 

 immersed in water contained in an iron tub. 



The weight of the granite ball was 22452'85 grams, and its mean 

 diameter was 25 1-46 millimeters. The length of the wire of suspen- 

 sion was 610-8 centimeters, and its diameter was 0889 millimeter. 

 The diameter of the iron tub was 2 ft. 4 in., and the depth of water 

 contained in it was 1 ft. 9 in. 



The method of observation was as follows : the indices of the ball 

 having arrived at the zero of rest, the ball was then displaced by a 

 torsional movement of the wire, and allowed to regain its position of 

 rest by a succession of vibrations of diminishing amplitudes. 



The quantities observed were, the time of vibration and the rate of 

 diminution of the amplitude. 



The equations of motion of the apparatus are thus found : — 



where x - the varying amplitude of any point of the surface of the 

 ball measured from the zero of rest ; 



X= the tangential forces of torsion and " drag" acting at the 

 point X. 



If we assume that for low velocities the friction will be proportional 

 to the velocity, we shall have 



X=P.-/|; (2) 



where h is a coefficient depending on torsion, and / is a coefficient 

 depending on " drag." 



^ By the term "Drag,'' I understand tlie combined effects of Friction and 



Tiscocity. 



