412 Reports on Special Researches 



land values. Some light is thrown on this disagreement when we observe that, on land, 



the values of the air-earth current-density have usually been measured with the Wilson 



electrometer/ while when the conductivity has been the main element sought, other types 



of apparatus have been more frequently used. There has been considerable diversity of 



opinion as to the proper method of usmg the Wilson electrometer, the uncertainty going so 



far as to have resulted in discussions of whether there should or should not be a factor of 2 in 



the formula used.^ 



SPECIFIC IONIC-VELOCITIES. 



We have very few ocean values of the specific velocities with which to compare the 

 present determinations. Practically the only data available are those of Knoche,^ who 

 found values in the neighborhood 0.05 cm. per second per volt per cm. in the Pacific Ocean; 

 but it would seem that unless the conditions were exceptional in Knoche's experiments, this 

 value must be subject to some doubt. The means of the land values as obtained from 

 Table 89 are v+ = 1.08 and v^ = 1.22. It is of interest to notice, however, that the present 

 ocean values v+ = 1.30 and i^. = 1.30 are in better agreement with laboratory values of the 

 specific velocities deduced from determinations on dust-free air than are the land values 

 measured in the open. Thus the values of v+ and v^ obtained by Zeleny, for ions produced 

 by Rontgen rays in moist air, are, at 14° C. and normal pressure, 1.37 and 1.51 cm. per 

 second per volt per cm. respectively.^ It is further of interest to notice that the ratio 

 v+/v- is practicaUy unity for the ocean values, whereas for the land values it is about 0.9. 



It is very probable that the difference between the land and sea values for the specific 

 velocities is attributable to the effect, on the measurements, of the so-called large ions 

 formed by the union of small ions with dust nuclei. The effect of these large ions is to 

 make the measured specific velocities of the small ions come out too small, and we should 

 thus expect the measurements to lead to smaller values for the specific velocity on land, 

 where there are many nuclei, than on the ocean, where there are few. In illustration of this 

 point, it may be remarked that at Kew, which is in the vicinity of the smoky atmosphere of 

 London, specific velocities are recorded as low as 0.5 cm. per second per volt per cm.'* It 

 would thus seem that the sea values are likely to be more accurately the representatives of 

 the true specific velocities of the small ions than are the land values. When one considers 

 the difficulties connected with ocean observations, the comparative constancy of the sea 

 values as shown by Tables 84 and 85 is very encouraging, and adds weight to the accuracy 

 of the detenninations of both conductivity and ionic content, which are the elements from 

 which v+ and V- are deduced. 



PENETRATING RADIATION. 



An examination of Tables 84 and 85 shows a remarkable constancy in the value of R, 

 the number of pairs of ions produced per c. c. per second in a closed vessel, and the mean 

 value 3.8 recorded in Table 85 is in general agreement with the results of Simpson and 

 Wright,® who found values of R ranging from 4 to 6 in the Atlantic and Indian Oceans. 



The values of R found over land are usually of the order of magnitude of 10 or more; 

 and the discrepancy between the sea and the land values is readily accounted for by the part 

 of the ionization which, in the case of the land values, is attributable to the T-ray radiation 

 from the radioactive material in the air and soil, and to the secondary /3-ray radiation which 

 this 7-ray radiation produces in the walls of the vessel. These som-ces of ionization are 

 practically absent in the case of the ocean measurements, for there is very little radioactive 

 material in the ocean or in the air over it. 



>See C. T. R. Wilson, Cambridge, Proc. Phil. Soc, vol. 13, pp. 363-382, 1906. 



^See G. Dob3on, London, Proc. Phys. Soc, vol. 26, pp. .334-346, 1914. 



'Phy3. Zeit., vol. 13, p. 325, 1912. 



*Phil. Trans. R. Soc. A, vol. 195, p. 193, 1900. 



'See E. H. Nichols, Terr. Mag., vol. 21, p. 94, 1916. 



'Proc. R. Soc. A, vol. 85, pp. 196-198, 1911. 



