December 20, 1917] 



NATURE 



307 



the ship rolling- through 30°, and even at times 

 40° or more. 



The magnetic observations made on shore are 

 discussed in separate tables, and there are exact 

 ■descriptions of the stations occupied. Consider- 



sea-deflector inside. 



able local disturbance was encountered in Madeira, 

 the Bermudas, St. Helena, Mauritius, and 

 especially in Iceland near Reykjavik. In such 

 cases several adjacent sites were occupied. 



The part dealing with atmospheric electricity 

 possesses many features of interest. Several new 

 instruments are described, one 

 for measuring- potential gradient 

 at sea. This was standardised 

 by means of simultaneous ob- 

 servations on shore when the Car- 

 negie was in harbour. The elec- 

 trical elements observed included 

 the potential gradient, the con- 

 ductivities arising from positive 

 and negative ions, the number 

 and mobility of positive and nega- 

 tive ions, the air-earth current, 

 the number of pairs of ions pro- 

 duced per c.c. per second in a 

 closed copper vessel, and the 

 radio-active contents of the air 

 and of sea-water. Regular 

 meteorological observations are 

 also included. The results ob- 

 tained are numerous and are 

 g^iven in tables on pp. 403-5. The 

 discussion of the results is accom- 

 panied by much information as to 

 the corresponding- data obtained tk,. 



by previous observers on land and 

 sea, and is practically equivalent to a text-book 

 on atmospheric electricity. 



The conclusions drawn are summarised on 

 p. 422, the following being perhaps the most 

 interesting-. The potential g^radient seems to be 

 NO. 2512, VOL. 100] 



much the same over the different oceans, the 

 mean daily value being about 113 volts per metre. 

 There is a distinct diurnal variation, of the same 

 general type as that for the year as a whole at 

 Kew, i.e. having two maxima and two minima, 

 but the principal maximum occurs 

 near midnight, i.e. two or three 

 hours later than at Kew. The 

 average numbers of plus and 

 minus ions per c.c. were respec- 

 tively 804 and 677, numbers very 

 similar to those encountered on 

 land. The mean value found for 

 the mobility was i'30 cm. /sec for 

 both plus and minus ions. The 

 mean value found for the air-earth 

 current was 95 x 10-'^ E.S.U. 

 The number of pairs of ions pro- 

 duced per c.c. per second in a 

 closed copper vessel shows little 

 variation over the ocean whether 

 with locality, season of the year, 

 or hour of the day. The mean 

 found was 3'8. The average 

 radium emanation contents in 

 curies per cubic metre of air found 

 over the Pacific and sub-Antarctic 

 oceans were respectively 3*3 x 

 10-12 and 0-4x10-12, the larger 

 of these values being only some 4 per cent, of the 

 average value over land. 



We learn that two more volumes, iv. and v. 

 of the series, are to deal with later observational 

 results, secular change, and the reduction of all 



-vii;w ot uie uriugc on the Catne^ic and observing-domes. 



the results, both for land and sea, to a common 

 epoch. It is hoped that complete world charts 

 based on these observations may be constructed 

 during 1918. 



C. Chree. 



