Discussion op Atmospheric-Electric Observations 413 



RADIOACTIVE CONTENT OF THE AIR. 



Former ocean measurements have concerned themselves with relative determinations, 

 by the Elster-and-Geitel method, of the radioactive content over sea and land. The 

 present method leads to an absolute determination of the amount of emanation in the 

 atmosphere, and gives the value 3.3 X 10"'^ curie per cubic meter for the Pacific-Ocean deter- 

 minations, and 0.4X10~^^ curie per cubic meter for the determinations in the sub- Antarctic 

 Oceans.^ The mean value for the whole cruise through March 1916, is 2.2X10~^^ curie 

 per cubic meter. Several absolute determinations of the radioactive content over land have 

 resulted in a mean value of 88X 10"^^ curie per cubic meter,'' so that the mean of the ocean 

 values forms only 2.5 per cent of that found on land. The results of the present cruise are 

 in general agreement with relative measurements over land and sea, in so far as one can 

 attach any meaning to these relative measurements. Thus, in terms of the arbitrary imit 

 defined by Elster and Geitel, Linke^ found 2.4 and I^oche^ 3.6 over the Pacific Ocean. 

 The mean of these values is 3.0. As typical values for land stations we have the following: 

 WolfenbutteP 19, Freiburg" 84, Karasjok' 93, Hochtal Arosa^ (Switzerland) 91, Altjoch am 

 Kochelsee' (upper Bavaria) 137. The mean of these activities is 85, of which the value 

 for the Pacific Ocean forms about 3.5 per cent. The mean Pacific-Ocean value of Q for the 

 Carnegie's fourth cruise forms 3.8 per cent of the land value, 88X 10"^^ curie per cubic meter, 

 so that the comparison of the absolute values on land and sea is in good agreement with that 

 of the corresponding relative values. 



As will be seen from Table 85, the radioactive content for the sub- Antarctic oceans is 

 much less than that for the Pacific Ocean. This result is in harmony with the experience of 

 Simpson and Wright,'" who call attention to the low value (3 Elster-and-Geitel units), for 

 the radioactive content observed by them along latitude 40° S., between the Cape of Good 

 Hope and Melbourne, as compared with the mean value (6 Elster-and-Geitel units) for 

 their whole cruise from England to Melbourne. The present values for latitudes 50°-60° S. 

 are, however, even smaller, in comparison \\ath land values, than are those of Simpson and 

 Wright for latitude 40° S., a point of considerable significance as suggesting a rapid diminu- 

 tion of the radioactive content with increase of southerly latitude. 



The small value of the radioactive content over the ocean is, of course, in line with 

 what might be expected when it is remembered that the radioactivity of the ocean air owes 

 its origin almost entirely to the emanation transported by winds from the land. Over the 

 small oceans, where the air has on the average passed more recently over land than is the 

 case with the air over the large oceans, comparatively large values of the radioactive content 

 are found. Thus, in the North Atlantic Ocean, Eve'^ found values of the radioactive 

 content of the same order of magnitude as those found over land ; Hewlett, on the Carnegie's 

 second cruise, found an activity of 12^^ Elster-and-Geitel units, while Johnston on the third 

 cruise found an activity of 23." 



As regards the effect of land on the radioactive content, Q, the results in Tables 80-83 

 are of considerable interest. The braces serve to divide the legs of the cruise into sections, 

 the number tabulated representing, for the respective section, the mean value of the radium- 

 emanation content in curiesX 10"'^ per cubic meter. Referring to Table 80,itwillbe observed 

 that there was a regular diminution in the emanation content as the Carnegie passed from 

 Balboa out into the Pacific Ocean en route for Honolulu. The voyage from Honolulu to 



'In obtaining the mean value for the sub-Antarctic *Phy3. Zeit., vol. 13, p. 112, 1912. 



oceans, the values of Q for May 30 and 31, 1916, have been 'See Elster and Geitel, Phys. Zeit., vol. 4, p. 526, 1903. 



omitted, since these were obtained when the yacht was quite 'See Albert Gockel, Phys. Zeit., vol. 5, p. 591, 1904. 



near the New Zealand coast and are obviously not represen- 'See G. C. Simpson, Phil. Trans. R. Soc. A, vol. 205, 

 tative of the general values obtained on the sub-Antarctic p. 61, 1905. 



voyage. 'See W. Saake, Phys. Zeit., vol. 4, p. 626, 1903. 



'See E. von Schweidler and K. W. F. Kohlrauseh, article 'See Elster and Geitel, Phys. Zeit, vol. 5, p. 11, 1904. 



on" Almospharische EleklriHldt," p. 223 (a section from vol. 3 '°Pi-oc. R. Soc. A, vol. S5, p. 186, 1911. 



of "Handbuch der Elektrizit.lt und des Majmetismus"). "Terr. Maj.. vol. 14. p. 25, 1909. 



^Geilingen Nach. Ges. Wiss., 1906. "See page 370. "See page 373. 



