CONTEMPORARY ADVANCES IN PHYSICS 165 



type appearing in equation (2), with definite and distinctive values of 

 M? For if so, we may affirm that formally the cosmic rays behave like 

 a mixture of gamma-rays having definite discrete frequencies, a sort of 

 line-spectrum. 



This is an extremely delicate question. What is needed is a method 

 like that of Fourier analysis, whereby a given curve may be resolved 

 into a sum, not of sines and cosines as result from the Fourier process, 

 but of terms of the type of the right-hand member of (2) ; it seems that 

 such a method is wanting. Millikan and Cameron, and independently 

 their colleague Bowen, "built up . . . our observed curve out of four 

 components — no smaller number would do — and in such a way that 

 the synthetic and the observed curve [that of 1931] fitted exceedingly 

 nicely from one to the other." These four components have the 

 values .0080, .0020, .0010, .0003 of the coefficient m; and their relative 

 intensities, as inferred to exist at the confines of the atmosphere before 

 the filtering commences, stand in the ratios 141000 : 130 : 80 : ZZ. 

 It is evident that the portion of the radiation which survives at great 

 depths of water, and for which the values of ix are extremely low, is a 

 very small part of all that is to be found in the upper air. Yet even 

 the highest of the values of ^ here cited is much lower than the coeffi- 

 cient of absorption for the hardest known gamma-rays from radioactive 

 substances.'^ 



It would be deplorable to leave unmentioned some very romantic 

 experiments in which the electroscope, with or without the observer, 

 ascended by the aid of a balloon to heights of air hitherto unattained. 

 Balloon-ascents were begun by German physicists (Gockel, Bergwitz, 

 Hess, Kolhorster) in the five years before the war. Before, it had 

 been maintained that the ionization of the air is due altogether to 

 radioactive substances in the ground. The earliest balloon-flights 

 impaired this argument, by proving that the amount of the ionization 

 does not diminish rapidly as the observer flies upward ; the later ones 

 destroyed it, by proving that above a certain level (variously stated, 

 and depending no doubt on the thickness of the walls of the chamber) 

 the ionization rises with increase of height. I will however speak 

 chiefly of more recent flights, those of Alillikan's apparatus and that 

 of Piccard. 



The electroscope of Millikan and Bowen was borne aloft above the 

 plains of Te.xas by a pair of balloons, one of which eventually burst and 

 left the other to serve as a parachute in lowering its burden gently 



J Though not higher than that of the gamma-rays excited by Bothe and Becker 

 when bombarding beryllium with alpha-particles, as I learn from a letter of Professor 

 Bothe. 



