i88 SCIENCE PROGRESS 



cess is only s %, and for all distances above 500 ft. the velocity 

 of the explosive sound from the largest gun is practically 

 normal. Careful observations of temperature, barometric 

 height, humidity, and wind velocity were taken over a maxi- 

 mum range of 21,000 ft. in order to obtain a value for the 

 velocity of sound under standard conditions. The final calcu- 

 lations of the corrections for this determination are not com- 

 plete, but a preliminary value of 1,089 ft. per sec. has been 

 worked out. Prof. Miller has also obtained photographs 

 of the wave form of sounds from large guns {Phys. Rev., 1920, 

 15, 230). It is found that in all cases the characteristics are 

 the same. There is no true vibratory form, but a single com- 

 pression pulse rising abruptly to a maximum, and then fall- 

 ing to a rarefaction of much smaller amplitude but longer 

 duration ; the whole disturbance passing any fixed point in 

 something of the order of a fiftieth of a second. Photographs of 

 air waves round a rifle bullet {Phys. Rev., 1920, 15, 518) show 

 that the bow and stern waves behind the bullet are quite 

 straight and parallel, and are propagated in the direction cor- 

 responding to the relative velocities of the projectile (2,720 ft. 

 per sec.) and of sound (1,123 ft. per sec. at the temperature 

 of the experiment). Near the projectile the bow wave increases 

 in velocity, probably owing to the heat developed by the pro- 

 jectile and by compression, until its velocity equals that of 

 the projectile itself. The stern wave, immediately behind 

 the projectile, has a diminished velocity, probably due to the 

 cooling produced by the rarefaction of the air ; this velocity 

 increases rapidly up to the normal velocity of sound when the 

 stern wave becomes parallel to the bow wave. The wake is 

 a very strongly developed and well defined turbulence which, 

 ten bullet-lengths behind the bullet, is about twice the diameter 

 of the bullet in cross-section. In the same number of the 

 Phys. Rev. (p. 516), A. T. Jones discusses the production of the 

 characteristic whine due to a shell. It is attributed to vortices 

 in the wake of the shell, and the author attempts to apply 

 Strouhal's laws for the seolian tones produced by wires to 

 calculate the variation in the pitch of the sound heard by an 

 observer standing in the plane of the trajectory. The results 

 obtained show no indication of any possibility of detecting 

 by ear whether the observer is nearly underneath the vertex 

 of the path, but in the case of an elevation of 45 '^ indicate 

 a rise of pitch followed by a fall. 



PHYSICAL CHEMISTRY. By W. E. Garner, M.Sc, University College, 

 London. 



The Reaction Limits of Mixed Crystals. — The investigation of 

 the chemical and galvanic properties of mixed crystals by 



