DEPARTMENT OF TERRESTRIAL MAGNETISM. 337 



In the case of all the electroscopes in the observatory, the measurements are 

 made by noting the time required for the fiber to travel between two fixed 

 marks on the scale, and the indications are reduced to volts in a moment by 

 calibrating systems which are permanently connected to each instrument and 

 are used in conjunction with a voltmeter common to all the systems. 



The paper concludes with a discussion of the results of the third cruise of 

 the Carnegie. (See next abstract.) 



The atmospheric-electric observations on the third cruise of the Carnegie, 1914. Report and 

 discussion by W. F. G. Swann. Terr. Mag., vol. 20, pp. 13-48 (March 1915). 



The atmospheric-electric observations discussed in the report comprised 

 the measurements made during the third cruise of the Carnegie while under the 

 command of Mr, J. P. Ault, in 1914. The general course of the Carnegie during 

 this cruise was as follows: Leaving Brooklyn on June 8, 1914, she arrived at 

 Hammerfest on July 3. Sailing again from Hammerfest on July 25, she 

 entered the harbor at Reykjavik, Iceland, on August 24, having reached the 

 latitude of 79° 52' north, off the northwest coast of Spitzbergen. Leaving 

 Reykjavik on September 15, the Carnegie arrived at Greenport, Long Island, 

 on October 12, returning to Brooklj^n on October 21, 1914. 



The observations in 1914 comprised, in addition to the magnetic and 

 meteorological data, measurements of the potential-gradient, the conductivities 

 for the positive and negative ions, and the radioactive content. Measure- 

 ments of the ionic numbers were also made during the passage from Greenport 

 through Long Island Sound to New York. All of the observations, with the 

 exception of a few measurements in Long Island Sound by the author, were 

 taken by Observer H. F. Johnston. 



The first portion of the report is devoted to a description of the methods 

 employed and to a general discussion of the sources of uncertainty. Measure- 

 ments of the potential-gradient were made by means of an ionium collector 

 attached to the end of a bamboo pole, and by the method described by the 

 author in a former communication.^ Measurements of the conductivity were 

 made by Gerdien's method. The ionic content was measured by a modifica- 

 tion of the Ebert ion counter and the radioactive content by a modification of 

 Elster and Geitel's method. 



The average value of the potential-gradient, atmospheric conductivity, and 

 radioactive content for the whole cruise were, respectively, 93 volts per meter, 

 2.52 X10~* E. s. u., and 23, the last number being expressed in Elster and 

 Geitel units. The average value of the Earth-air current for the whole cruise 

 was 7.7X10""^ E. s. u. per square centimeter. 



The atmospheric-electric elements were measured daily between the hours of 

 9 a. m. and 12 noon. The observations as far as they go indicate a general 

 increase of the potential-gradient from summer to winter, which is in accord 

 with land observations for the daily mean values. The conductivity also 

 shows a general increase from the beginning of the cruise (June 8, 1914) to 

 about the end of September, when a maximum occurs, after which the conduc- 

 tivity falls; the Earth-air current-density follows the general course of the 

 conductivity. No very definite conclusions evolve as to the seasonal varia- 

 tions of the radioactive content, though the results are not inconsistent with 

 those of Simpson in Lapland, in indicating a higher active content in winter 

 than in summer. Table 4 shows the mean values of the various elements 

 arranged according to the period as given in the first column, X4- and X_ referring, 

 respectively, to the conductivities for positive and negative ions. The quan- 

 tity 1) in the last column is proportional to the radioactive content of the 

 atmosphere. 



^Terr. Mag., vol. 19 182-185, September 1914. 



