414 Rkports on Special Researches 



Dutch Harbor (Table 81) took place for the most part in a region of the ocean far removed 

 from land, and Q was small. For the voyage from Dutch Harbor to Port Lyttelton, Table 

 82 shows a decrease in Q as the land regions in the vicinity of Alaska were left behind, and 

 in the parts of the cruise between 0° and 40° north latitude, where the Carnegie was very far 

 from land, Q was very low. The value, however, increased again as the land regions formed 

 by Australia and New Zealand were approached. Attention has already been called to 

 the low values obtained for Q in the sub-Antarctic voyage. The values here were naturally 

 variable, for the wind directions which are fruitful in bringing emanation to any point of 

 the sub-Antarctic oceans are very limited in extent. This circumstance is emphasized by 

 the fact that the ice-covered Antarctic Continent probably does not contribute appreciably 

 to the emanation content of the winds coming from it. 



Since a radium emanation content of 10~^- curie per cubic meter is sufficient to account 

 for a rate of production of 0.021 ion per c. c. per second,^ the average amount of radium 

 emanation over the Pacific and sub- Antarctic Oceans, as determined by the results of the 

 present cruise, is capable of accounting for the production of about 0.05 ion per c. c. per 

 second. 



CAUSES OF ATMOSPHERIC IONIZATION OVER THE OCEAN. 



If we assume the well-known relation q = an^, between n, the number of ions per c. c. 

 of either sign, q the rate of production, and a the coefficient of recombination of ions, we can 

 find the value of q necessary to account for any assigned value of n. The n which figures in 

 the relation q — an- is somewhat greater than the values of either /i+ or n_ as measured, since 

 the measured values are in part determined by the influence of the potential-gradient in 

 altering the ionic distribution near the Earth's surface.^ Thus, referring to Table 85, we 

 see that according to the results of the present cruise a value of n at least as great as 800 

 must be accounted for. Further, there is in the atmosphere a class of ions, the so-called 

 large ions, for which the specific velocity is only about 1/3000 of that of the small ions. 

 These large ions are not measured by the ion counter, but the fact of their existence increases 

 the rate of production of ions which it is necessary to postulate in order to account for 

 atmospheric ionization. In order to account for the 800 small ions alone, we find, taking 

 a = 2.5 X 10"^ that q must have a value equal to 1 .6. Hence the radioactive material in the 

 air over the great oceans is only sufficient to provide for about 3 per cent of the rate of 

 production of ions necessary to account for the presence of the small ions alone. It is true 

 that the uncertainties inherent in the method of determining the emanation content from 

 the active deposit are such as to give a value which is too low, but after making all allow- 

 ances for such considerations, we can not but conclude that the radioactive content over the 

 large oceans is too small to play an important part in controlling the ionization of the 

 atmosphere there. A sunilar remark applies to the radium emanation contained in the sea- 

 water, which is likewise known to be excessively small, so that there remains, for the 

 explanation of the bulk of the atmospheric ionization over the ocean, only that source, 

 whatever it is, which is responsible for the formation of ions in a closed vessel. 



As already stated, the results of the Carnegie's fourth cruise indicate a value 3.8 for 

 the rate of production of pairs of ions per c. c. in a closed vessel. If we could assume all of 

 the ions produced in a closed vessel over the sea to have their origin in causes other than the 

 vessel itself, we should be provided with a source of ionization more than sufficient to 

 account for the existence of the small ions; but the question as to how far the ioniza- 

 tion produced in a closed vessel of this kind is really the result of actions other than 

 that of radioactive impurities in the vessel is still to some extent an open one. 



'For the theoretical basis underlying this calculation see E. von Schweidler and K. W. F. Kohlrausch, article on 

 " Atmosphdrische Elektrizital," p. 234 (a section from vol. 3 of "Handbuch der Elektrizitat und des Magnetismus"). 

 ^See E. von Schweidler, Wien. Ber.. vol. 117, p. 653, 190S; also W. F. G. Swann, Terr. Mao-, vol. 18, p. 163, 1913. 



