DEPARTMENT OF TERRESTRIAL MAGNETISM. 329 



greater than the land values and are, moreover, more nearly equal to each 

 other, and to the values obtained in laboratory experiments made with dust- 

 free air. This result is what might be expected in view of the purity of the 

 ocean air. The values of R (see table 5) show a remarkable constancy, and 

 are in good agreement with the results of Simpson and Wright, who found 

 values of this quantity in the south Atlantic and south Indian Oceans ranging 

 from 4 to 6, 



The mean value of the radium-emanation content for the whole cruise 

 forms only about 2 per cent of the average value found over the land, and is 

 far too small to contribute in any marked degree to the ionization over the 

 ocean. It is, nevertheless, of interest to remark that the difference between 

 the radium-emanation contents for the Pacific and sub-Antarctic Oceans is 

 in the right direction, and of the right order of magnitude, to account for the 

 slight difference in the ionic contents observed over these oceans. The 

 emanation content, though small, shows, throughout the cruise, a very 

 decided variation with the distance from land and with the wind direction, 

 the variation being of the type to be expected on the view that the land is 

 the primary source of the emanation. 



In order to explain the ionic content over the ocean it is necessary to account 

 for a rate of production of about 1.6 ions per cubic centimeter per second. 

 The source responsible for the ionization in a closed vessel is amply sufficient 

 to provide for the necessary rate of production of ions, if we are justified in 

 attributing an appreciable fraction of this ionization to causes other than 

 the vessel itself. The radioactive material in the soil and in the air over the 

 land are, however, sufficient to account for a rate of production of 4.5 ions 

 per cubic centimeter per second, so that the main difficulty resulting from a 

 comparison of the ionic densities over land and sea is to be found not so 

 much in accounting for the latter as in accounting for the fact that the 

 ionization over land is not much greater than that over the sea. It would 

 seem that the explanation of the difficulty must be sought in the slowly 

 moving ions (large ions) formed by the union of the so-called small ions with 

 dust nuclei. The large ions are not measured by the ion-counter, but they 

 have to be maintained, since they are continually suffering recombinations. 

 It turns out that, if we assume the number of large ions over the ocean to be 

 insignificant, it is necessary to assume for the land a number of large ions 

 per cubic centimeter about equal to the number of small ions per cubic centi- 

 meter in order to account for the fact that the measured ionic density over 

 the land is no greater than that over the sea. 



The potential-gradient shows a distinct diurnal variation, and the mean 

 diurnal variation curve for the whole cruise gives minima about 5 a. m. and 

 3 p. m., and maxima about 9 a. m. and midnight. These results are in gen- 

 eral agreement with those of Simpson and Wright, who, in 1910, made a few 

 determinations of the diurnal variation of the potential-gradient in a region 

 of the ocean Avithin 40° of the Equator. The Fourier analysis of the diurnal- 

 variation curve for the Carnegie's fourth cruise shows the amplitude of the 

 12-hour "wave" to be greater than that of the 24-hour "wave," a result of 

 considerable importance in connection with the theory of the origin of the 

 former. 



The rate of production of ions in a closed vessel shows no appreciable 

 diurnal variation; the ionic content, however, shows a distinct variation, with 

 a flat maximum extending from about 6 a. m. to 2 p. m., and a minimum at 

 midnight. The diurnal variation of the ionic content is of the type to be 

 anticipated from the secondary influences resulting from the diurnal varia- 

 tions of potential-gradient, and temperature. 



