72 Art. 9.-T. Terada: 



investigated by H. Lamb and also by Prof. S. Sano.'^ Both 

 authorities agree in the result that the period must be roughly 

 of the order of ö minutes, if we assume a plane earth and take 

 the ordinary value of the velocity of sound, which is assumed 

 to be uniform throughout the atmosphere. According to S. 

 Sano, the period is given by 



y_ 47rc 

 Ï0 



where c is the velocity of sound and ?'=1'41. Since c=VrBd, i.e. 

 the period increases witb the temperature, we must assume a 

 lower temperature on the side of the upper atmosphere facing 

 the sun than on the opposite side. This seems at first sight 

 strange, but is in accordance with the fact that the diurnal 

 variation of temperature in the upper layer shows a tendency to 

 become opposite in phase compared with the lower layer'; 

 though the amplitude is of course small, a tendency is suspected, 

 that it increases with height. The diflficulty is to explain the 

 actual amount of the difference of the characteristic periods, the 

 niglit-wave being nearly twice as long as the day-waves. The 

 absolute temperature must then be assumed nearly 4 times 

 higher on the night side, if the difference of the periods is to be 

 exclusively attriJnited to the difference of IK If the corpuscular 

 radiation from the sun chiefly frequents the night side of our 

 atmosphere, as is supported by different phenomena pertaining 

 to aurora and magnetic storms, the effective temperature of that 

 side of the atmosphere may be raised considerably by the presence 

 of the additional kinetic energy of the free electrons. As to the 

 increase of c due to the ionizations, no conclusive experimental 

 evidence is yet afforded,^' especially for the highly rarefied state 



1) H. Lamb, Proc. London Math. Soc, '2] 7, 1907, p. 122. S. Sano, Bull, of the Central 

 Meteor. Obs., Japan, 2, No. 2, 1913. 



2) T. E,eo-er, Arbeiten d. kön. preuss. aeronautischen Observatoriums bei Lindenberg, 

 S, p. 229. 



8) W. Küpper found a sensible increase of c for gases ionized by radiations of different 

 kinds, Dissertation, Marburg, 1912 ; Ann. d. Phys. [4] 43, 1914, p. 905. W. H. Westphal 

 obtained, however, a negative result. Verb. d. deutsch, phys. Ges , 1914', p. (313. These results 

 refer to gases at ordinary pressure. 



