334 OBSERVATIONS OF ATMOSPHERIC ELECTRICITY. 



the rarer and rarer air above us being a non-conductor, and the so-called vacu- 

 ous space, or the interplanetary space beyond that (which we cannot admit io 

 be really yacuous), being a non-conductor also, then a charge could be given 

 to the earth as a whole, if there were the other body to come and go away 

 again, just as a charge would be given to a pith-ball electrified in the air of 

 this room. Then, I say, all the phenomena brought to light by atmospheric 

 electrometers, which we observe on a fine day, would be observed just as 

 they are. The ordinary observations on atmospheric electricity are pre- 

 cisely the same as if the the earth were electrified negatively, and the air 

 had no electricity in it whatever. In rainy weather, however, the potential 

 of the atmosphere referred to that of the earth is sometimes positive and some- 

 times negative. 



Observations made everywhere in the northern hemisphere tend to show 

 that the potential is greater in summer than in winter, but the months of 

 maxima and minima ajDpear to differ at different places. Observations made 

 at Kew and Windsor in JSTova Scotia show distinctly tw^o maxima in the^ 

 year, those at Brussels and Kreuznach only one. Both the Kew and Brus- 

 sels observations show two maxima daily, at 8 a. m. and 10 p. m, in July, at 

 10 a. m. and 7 p. m. in January, and at about 9 a. m. and 9 p. m. in spring 

 and autumn. Although, therefore, all the tests made at different parts of 

 the earth's surface in fair weather (except some of doubtful meaning made 

 at the peak of Teneriffe in the early days of the study of this question) have 

 shown the earth's surface to be negatively electrified, the amounts of elec- 

 tricity existing at the same time at different places will be very different ; 

 and this difference manifests itself in a manner often extremely disagreeable 

 to the telegraph engineer — in natural line currents. 



The country in which these natural line currents have been most care- 

 fully studied is undoubtedly British India, since the uniform system of land 

 line testing employed in the Government Telegraphs throughout that coun- 

 try causes the accurate measurement of these currents to be daily carried out. 

 From the results of 10,000 such measurements it is seen that in India the 

 direction of the current is far more constant than its magnitude, and on the 

 whole there appears to be a marked preponderance of currents ot positive 

 electricity flowing from the East to the West — that is, with the sun ; and 

 such a current the laws of electro-magnetism tell us would be consistent 

 with the earth's magnetism. 



Observations made on the Atlantic cables tend to show that when there 

 are no unusual disturbances the earth currents at one end have two positive 

 maxima and two negative maxima daily. Submarine cables, however, even 

 when long, are far less disturbed by terrestrial currents than land lines,. 

 which may possibly be due to the sea having a far greater electric conduc- 

 tivity than the land. 



Since the early days of telegraphy a large number of observations of nat- 

 ural currents have been made at the principal London Office in Telegraph 

 street, the results of which were communicated to the Astronomer-Eoyal. 



