186 



SCIENCE 



[N. S. Vol. XLIX. No. 1260 



fereat from, but probably somewhat less than, 

 twelve mean solar hours. 



Hence any cause of pressure cliange, having 

 a semidiurnal period or approximately so, 

 would, if of sufficient magnitude and proper 

 phase, account for the twelve-hour barometric 

 curve. All that is needed, apparently, to give 

 the .semidiurnal pressure curve is a pressure 

 impulse of the same period, twelve hours, as 

 that of the free vibration of the atmosphere as 

 a whole. And this is furnished by the forced 

 forenoon barometric maximum, followed six 

 hours later at the same place by the forced 

 afternoon barometric minimum. In other 

 words, taken together, the forenoon and after- 

 noon forced disturbances appear to occur with 

 the proper time-interval necessary to set up 

 and maintain the twelve-hour free vibrations 

 of the atmosphere. 



The course of events at each locality ap- 

 pears to be substantially as follows: 



1. A forced forenoon compression of the 

 atmosphere, followed by its equally forced 

 afternoon expansion, the two together forming 

 one complete barometric wave, with a ten 

 o'clock maximum and a four o'clock min- 

 imum, in harmony with the free vibration of 

 the entire atmospheric shell. 



2. ISTon-disturbance through the night or 

 during the time of a single free vibration. 



3. Eepetition the following day of the forced 

 disturbance in synchronism with, and there- 

 lore at such time as to reenforce, the free 

 vibrations. 



The series of disturbances is continuous, 

 forced by day and free by night, but the re- 

 sulting amplitudes of the barometric changes 

 are limited, through friction and through 

 the absence of perfect synchronism, to com- 

 paratively small values. Each point upon the 

 atmospheric sheU receives at every alternate 

 swing a forced impulse in phase with the free 

 vibration, and therefore at such time and in 

 such manner as indefinitely to maintain the 

 vibrations of the atmosphere as a whole. 



The forenoon maximum and the afternoon 

 minimum are primary disturbances equally 

 forced but in diSeremt ways by the daily in- 

 crease of temperature, while the evening max- 



imum and the morning minimum are second- 

 ary disturbances caused by the joint action of 

 the forced primaries through the twelve-hour 

 free vibration of the atmosphere. In short, 

 the semidiurnal swing of the barometer is a 

 result of merely fortuitous circumstances — of 

 the fact that the mass of the atmosphere hap- 

 pens to be such that the period of its free 

 vibration is approximately just one half that 

 of the earth's rotation. 



ATMOSPHERIC ELECTRICAL PHENOMENA 



The selected contributions to the physics of 

 the air just reviewed belong to the domains 

 of mechanics and thermodynamics. But there 

 have also been recent contributions to other 

 branches of the subject, especially to atmos- 

 pheric electricity, among them, 



(a) The discovery in 1900 by C. T. E. Wil- 

 son and, also, by H. Geitel of spontaneous 

 ionization in the atmosphere. 



(6) The discovery in 1902 independently by 

 Eutherford and Cook, and by McLennon and 

 Burton, of a penetrating radiation in the 

 lower atmosphere, presumably from radioac- 

 tive substances near the surface of the earth. 



(c) The discovery in 1905 by Langevin of 

 slow moving or large ions in the atmosphere. 



(d) The discovery by Simpson in 1908 and 

 1909 that the electric charge on thunderstorm 

 rain, and precipitation generally, is prevail- 

 ingly positive. 



(e) The discovery in 1908, also by Simpson, 

 of the probable origin of the electric charge of 

 the thunderstorm. 



(/) The discovery by Kolhorster in 1914 of 

 an extremely hard or penetrating radiation in 

 the atmosphere that seems to be of extra 

 terrestrial origin. 



Each of these several discoveries has its own 

 peculiar interest, but the origin of tlie electric 

 charge of the thunderstorm involves more 

 meteorological phenomena than does any of 

 the others, and, therefore, it is selected for 

 further remarks. 



Many have supposed that, whatever the 

 genesis of the thunderstorm, the lightning, at 

 least, is a product or manifestation of the 

 free electricity always present in the atmos- 



