DEPARTMENT OF TERRESTRIAL MAGNETISM. 269 



Reductions of field observations have been kept up to date as far 

 as time permitted, chiefly by Messrs. Fisk and Ault. Special instruc- 

 tions, directions, and compilations of data were prepared for the 

 Crocker I.and Expedition. The observations and magnetic results 

 obtained by the Australasian Antarctic Expedition have been revised 

 and prepared for publication. Detailed studies and discussions have 

 been made of the performance of specially made watches submitted 

 by five prominent manufacturers for use in the field. Articles 

 describing instruments manufactured by the Department have been 

 jDrepared by Messrs. Bauer, Dorsey, Fleming, and Widmer and pub- 

 lished (see BibliographjO • 



Dr. Swann, after an examination of various matters submitted to 

 him in relation to the past work of the Department in atmospheric 

 electricit}', had his attention turned to the subject of atmospheric 

 electricity in general, especially in relation to ocean measurements. 



Certain pieces of apparatus and certain methods have been devised 

 which he hopes to submit soon to experimental trial and which are 

 intended primarily to facilitate measurements of atmospheric elec- 

 tricity at sea, more particularly with a view to overcoming difficulties 

 due to the motion of the ship. A new method of determining the 

 atmospheric potential gradient has been proposed by him which 

 involves measuring the velocity which it is necessary to give to a 

 group of electrons to enable them to travel a fixed distance against 

 the Earth's field. The method, if successful in application, will have 

 manj^ advantages over the usual one, more particularly in the instan- 

 taneousness of its action. 



While considering the question of the measurement of the atmos- 

 }^heric potential gradient, Dr. Swann investigated a theory as to the 

 connection of this quantity with certain other atmospheric electrical 

 phenomena. In connection with this work, there was occasion to 

 examine the usual theory of conduction in a gas between two parallel 

 plates, and it was found that the assumptions made in the calcula- 

 tions are equivalent to imagining the plates to be an infinite distance 

 apart. The corrections involved, even when the plates are close 

 together, are not considerable, unless the rate of production of ions is 

 small, for which case, however, they may become enormous; conse- 

 quently an amended calculation of this important problem has been 

 developed.* 



INSTRUMENTAL WORK. 



An investigation of methods used in the determination of the 

 moment of inertia of magnets was made by Dr. N. E. Dorsey, who 

 also specially examined and prepared a report on the determination 

 of the moment of inertia of magnet H26 used at the Cheltenham 

 Magnetic Observatory. This discussion has shown that the greater 



*For fuller publication, see journal Terrestrial Magnetism and Atmospheric Elec- 

 tricity for December 1913. 



