728 Dr. Gr. A. Shakespear : Experiments on the 



In concluding, the writers desire to call attention to the 

 fact that this is, they believe, the first time that straight 

 physical analogy has been applied to the complex physio- 

 logical-psychological problem of flicker. 



It is our intention to treat in a subsequent paper the case 

 of unequal exposures of the two coloured lights under com- 

 parison, for whieh we have a theoretical treatment completed, 

 but for which experimental data are lacking. 



Physical Laboratory, United Gas Improvement Co., 

 Philadelphia, Pa. 



LXXV. Experiments on the Resistance of the Air to Falling 

 Spheres. By G. A. Shakespear, M.A., D.Sc* 



rriHE experiments here described were for the most part 

 JL carried ont between January and August, 1911, and a 

 note of the results was given at the Birmingham meeting 

 of the British Association in 1913. It was hoped to extend 

 their range by obtaining spheres of greater and less 

 diameters than those actually used, but hitherto I have not 

 found it possible to secure these. The present paper gives 

 an account of the method used. 



The principle of the method is simple. A hollow sphere 

 of celluloid was allowed to fall from various heights in the 

 Chamberlain tower of the Birmingham University, the time 

 of fall being noted for each height. The heights used were 

 770 cm., 1456 cm., 2142 cm., 2828 cm, 3800 cm. When the 

 effective density of the sphere was not too great, the 

 terminal velocity was reached at each of the last two 

 distances and was equal to the ratio of the difference of 

 these distances and the difference of the times of fall. It 

 was assumed that at this velocity the resistance of the air 

 was equal to the weight of the sphere in air. The effective 

 density of the sphere was varied by introducing into it 

 different quantities of small lead shot. In this way the 

 resistance at different velocities was found. 



In order to find how the resistance depended on the 

 diameter of the sphere, similar spheres of different diameters 

 were loaded so as to fall 3800 cm. in equal times, as observed 

 by letting them fall simultaneously on to a calico sheet 

 stretched horizontally near the floor. 



Method of Timing the Fall. — This was done with an 

 electrically maintained tuning-fork, which inscribed by means 

 of a metallic style a sine curve on a rotating drum covered 



* Communicated by the Author. 



