214 ANNUAL REPOKT SMITHSONIAN INSTITUTION, 1928 



scions of any particular instant at which the dawn came. Since the 

 days of Greek mythology, very few discoveries have sprung full- 

 grown out of the brain of anyone. 



EARLY WORK ON COSMIC RAYS 



A starting point in the study of the very " penetrating radiations "' 

 near the earth's surface was made in 1903, when these rays were 

 brought to light and named by McLennan,'' Rutherford,^ and their 

 collaborators, who found that the rates of discharge of electroscopes 

 could be very markedly reduced by surrounding them with successive 

 screens of lead several centimeters thick, thus showing that rays 

 existed in the atmosphere capable of penetrating such thick screens, 

 and therefore appropriately named the " penetrating radiations." 



The next important step was taken by the Swiss physicist Gockel,^ 

 who in 1910 first took an inclosed electroscope in a balloon to a height 

 of 4,500 meters and found, contrary to expectation, that the radiation 

 was higher at this altitude than at the surface, a fact which at once 

 suggested that all of it, at least, was not of terrestrial origin, but 

 that a part of it came into the earth's atmosphere from above, an 

 idea which had been put forward by O. W. Richardson '^ so early as 

 1906. 



During the next four years, Hess ® in Austria and then Kolhorster^ 

 in Germany made other flights like Gockel's, checked his results, and 

 rendered them more quantitative, Kolhorster taking balloon readings 

 up to 9,000 meters and finding the discharge rate decreasing slightly 

 up to about 1,000 meters and then increasing, until at 9,000 meters it 

 was some seven times as great as at the surface — more accurately, 

 80 ions more than at the surface, since it was this difference which he 

 reported rather than the readings themselves. 



The war put a stop for a while to further advances, but in the 

 fall of 1921 and the spring of 1922 Millikan and Bowen i° took the 

 next important step by building and sending up recording electro- 

 scopes in sounding balloons to a height of nearly 10 miles — 15,50C 

 meters — more than nine-tenths of the Avay to the top of our atmos- 

 phere, measured by the fraction of the air left below. These flights 

 checked the results of the European observers in indicating an in- 

 creasing discharge rate up to that height, though the new observed 

 rate was very much less than had been computed from the afore- 

 mentioned observations up to 9,000 meters; thus showing that the 



< McLennan and Burton, Tliys. Rov., IG, 184; 1003. 



B Rutherford and Cooke, Thys. Rev., 16, 183; 1003. 



"Gockel, Phys. Zeit., 11, 280; 1010. 



" Richardson, Nature, 73. 607 ; 74, 55 ; 1906. 



8 Iless, Phys. Zelt., 12, 908 ; 1011 ; and 13. 1084 ; 1912. 



B Kolhorster, I'hys. Zeit., 14, 1153; 1913; and Verh. d. Deut. Phys. Ges., .July 30, 1914. 



"Millikan and Bowen, Phys. Rev., 22, 198; 1923; and 27, 353; 1926. 



