96 Mr. E. A. Milne on 



The initial air-content o£ the water in the two bulbs may 

 be taken as practically 0. 



Sea-water when saturated with air at 13° C. contains per 

 litre : — 



Oxygen 6*06 c.c. 



Nitrogen , 11*77 c.c. 



During the experiment 7*935 grams water were evaporated. 



It will be seen from the above figures that, while the 

 water in the upper bulb was only a little more than half 

 saturated, that in the lower bulb was also very nearly half 

 saturated. That is, the air-content of the water was 

 practically uniform, although the surface-layer was not. 

 saturated. And the figures consequently prove that, while 

 the dissolved gases were freely drawn down the connecting 

 tube into the lower bulb by gravitation, they showed no 

 tendency to spread in lateral directions in the upper bulb 

 during their downward passage through it *. 



The authors desire again to express their indebtedness to 

 Dr. Hackett for the interest that he has continued to take 

 in their investigation of the subject of this and previous 

 communications. 



VI. Sound Waves in the Atmosphere. By E. A. Milne, 

 J3.A., Fellow of Trinity College, Cambridge \. 



ri^HIS paper deals with the kinematics of the propagation 

 JL of sound waves through a medium (such as the atmo- 

 sphere) in which the velocity of the medium and the velocity 

 of sound in the medium vary from point to point. The subject 

 came before the writer during the war, when he was working 

 as a member of: the Anti-Aircraft Section of the Munitions 

 Inventions Department under Prof, (then Major) A. V. Hill, 

 in connexion with the location of aircraft by sound. The 

 direction determined by a direction-finding instrument is not 

 in general that of the source ; corrections are required for 

 the convection and refraction undergone by the sound waves 

 in their passage through the atmosphere. The necessary 

 approximate formulas (for the case of an instrument deter- 

 mining the normal to the incident waves) were first given 

 by Hill. The present investigation was originally started 

 to examine in detail the errors in Hill's approximation, but 



* See Phil. Mag. March 1905. 



t Communicated by Prof. A. V. Hill, Sc.D., F.E.S. 



