PRESIDENTIAL ADDRESS, S.T.E. 1874. 231 



too much, then, when I say that the statement 

 that the air is positively electrified has been at all 

 events a subject for ambiguous and contradictory 

 propositions ; in fact, what we know by direct 

 observation is, that the surface of the earth is 

 negatively electrified, and positive electrification 

 of the air is merely inferential. Suppose for a 

 moment that there were no electricity whatever 

 in the air that the air was absolutely devoid of 

 all electric manifestation, and that a charge of 

 electricity were given to the whole earth. For 

 this no great amount would be necessary. Such 

 amounts as we deal with in our great submarine 

 cables would, if given to the earth as a whole, 

 produce a very considerable electrification of its 

 whole surface. You all know the comparison 

 between the electro-static capacity of one of the 

 Atlantic cables, with the water round its gutta- 

 percha for outer coating and the earth with air and 

 infinite space for its outer coating. 1 Well now, if 

 all space were non-conducting and experiments 



1 The earth's radius is about 630 million centimetres, and its 

 electrostatic capacity is therefore 630 microfarads or about that of 

 1, 600 miles of cable. 



