[maass] 



MEASUREMENT OF SURFACE TENSION 



137 



with a sine curve in the limit: ampUtude tending to zero. Since the 

 amplitude was very small in all the experiments, curve ABC may 

 be written 



Where D is the diameter BE of the wave crest and therefore r the mean 

 radius of the wave, and a the amplitude. 



To measure the velocity of a wave we must know its mean cross- 

 section. The latter is equal to 



A «^ o 



Substituting (4) for y and integrating, we get as the mean cross 

 section 



TrTr^-la^^ 



(5) 



Now, in all measurements, a was very small compared with r, 

 so that «^ may be neglected when compared with r^. Hence the 

 mean velocity is given by 



W 



7rr 



(6) 



where W stands for the number of cc. flowing through the stream per 

 second. 



D and d were measured on the negative plate in a manner similar 

 to that by which the wave-length was measured; the cross-hair being 

 placed tangent to each wave crest or wave hollow and the distance 

 to the corresponding wave crest or wave hollow being measured. 



The temperature of the liquid was measured in the outflow. 



Figures 9, 10 and 11 show enlarged photographs of streams for 

 which results are given. 



The last column gives the mean value of T for each stream ; when 

 the three values are reduced to 12°C these become 74-40, 74-80, 



