DEPARTMENT OP TERRESTRIAL MAGNETISM. 



327 



account and discussion by W. F. G. Swann of the results obtained on this 

 cruise. The general account of the atmospheric-electric equipment and of 

 the methods of measurement employed is sufficiently covered in the abstract 

 of W. F. G. Swann's paper "The atmospheric-electric work of the Depart- 

 ment of Terrestrial Magnetism," given on pages 334-337 of the 1915 Annual 

 Report published in Year Book No. 14. (For details of cruise, see pages 

 317-318 of the same report and pages 294-303 of present report.) 



The observations from New York to Colon were made by S. J. Mauchly 

 and H. F. Johnston. From Balboa (April 12, 1915) until the return of the 

 vessel to Lyttelton, New Zealand, on April 1, 1916, after her sub-Antarctic 

 voyage, they were made by Observers H. F. Johnston and I. A. Luke, and 

 thereafter Observers B. Jones and I. A. Luke carried on the work. The 

 following are the atmospheric-electric quantities measured : (1) The potential- 

 gradient X; (2) the conductivities (X+ and X_) arising from the positive and 

 negative ions; (3) the numbers (n+ and n_) of positive and negative ions per 

 cubic centimeter; (4) the number (R) of pairs of ions produced per cubic 

 centimeter per second in a closed vessel; (5) the radioactive content of the 

 atmosphere; and (6) the radioactive content of the sea-water. 



Table 4. — Mean values of atmospheric-electric elements, uncorrected for diurnal variation. 



The meteorological observations which are made are : pressure, temperature, 

 humidity, extent and nature of clouds, wind strength and direction. 



The diurnal variations of the potential-gradient, ionic content for the posi- 

 tive ions, and penetrating radiation are also under investigation. The specific 

 ionic velocities v+ and v- for positive and negative ions are given by the 

 relations v+ = \+/n+e, and y_=X_/w_e, respectively, e being the electronic 

 charge, and the air-earth current-density i is obtained from the relation 

 t = (X+4-X_)X 



The mean values of the atmospheric-electric quantities for each leg of the 

 cruise are recorded in table 4, the numbers of days on which observations 

 were taken being recorded in parenthesis under each value given. The last 

 column gives the radium-emanation content in curies XlO"^^ per cubic 

 meter. The results for the potential-gradient, ionic content, and conduc- 

 tivity in table 4 all correspond approximately to the time 9^.6. Of the values 

 from Brooklyn to Colon, more than half were obtained in the landlocked 



