i879-J ^ New Theory of Terrestrial Magnetism. 289 
electricity? To produce the earth’s magnetism we must have, 
in accordance with the known laws of eleCtro-magnetism, a 
negative current flowing from west to east or in the direction 
of rotation of the earth. In the language of the new theory, 
therefore, the surface of the earth must be negatively charged, 
but Sir William Thomson has proved, by observations with 
his electrometer, that all the phenomena brought to light by 
atmospheric electricity, on a fine day, would be observed just 
as they are if the earth had a negative charge. 
Lastly, in order to get a rough approximation of what sum 
be the difference of eleCtric potential between interplanetary 
space and the earth so that its own eleCtric charge alone 
combined with its known rate of rotation round its axis shall 
produce the earth’s magnetic moment, as determined by 
Gauss, Profs. Perry and Ayrton take as an example a solid 
sphere of iron of the size of the earth, and rotating with the 
earth’s known rate of rotation, and they prove mathemati- 
cally that something like a difference of potential a hundred 
million times the electromotive force of a single Daniell’s 
cell would be sufficient for this purpose. They notice 
that there is no difficulty in imagining such a dif- 
erence of potential to exist between the earth and inter- 
planetary space, seeing that there exists between the 
earth and the planets an enormous region of space having 
an insulation far higher than that of such a vacuum as ex- 
periment has shown will not allow the passage, from one 
point to another very near it, of a spark produced by a large 
induction coil. 
Lastly, they draw attention to the faCt that they have 
assumed the distribution of the eleCtric charge on the earth’s 
surface to be uniform, and so have arrived at a distribution 
of magnetic intensity dependent only on the latitude of the 
place, but that since it is possible that the sun and planets 
may have potentials differing immensely from that of the 
earth, it might be expeCted that the distribution of eleCtric 
charge on the earth’s surface, and consequently the magnetic 
intensity, would have variations like those of the tides, in 
faCt such variations as are known to exist ; also, that it 
would be anticipated that any sudden formation of vapour 
on the earth’s surface, or alteration of the sun’s atmosphere, 
or anything causing change in the lines of static electric 
induction from the sun to the earth ought to cause disturb- 
ances such as we know as magnetic storms. The alignments 
of planets with the sun and earth, again, or the proximity of 
planets to the sun having less than the average difference of 
eleCtric potential from it, ought to diminish the disturbances 
