121 
on investigation, that the quantity in question, if it has a 
diurnal variation at all, must vary on each circle of latitude 
with the solar time of some particular meridian, and not with 
local time. There can only be a very small fraction of the ob- 
served diurnal variation changing in this fashion. We may 
then say that, assuming the variation of the westerly compo- 
nent of magnetic force to be known over the surface of the 
earth, the knowledge of the northerly component will follow. 
This is of importance, considering the ease with which changes 
in declination are observed compared to change of horizontal 
force. Nevertheless, changes of horizontal force ought to 
be observed, wherever possible, as the two records at any 
one station will be equivalent to the record of declination 
only at two stations. 
Moreover, we cannot assume without proof that the mag- 
netic changes of diurnal variation are subject to the existence 
of a potential. If there is any actual discharge through the 
earth’s surface it will not be the case, and if there is only a 
variation of electric charge it would be equivalent to an 
electric current. 
The following calculations will show the order of magni- 
tude of the vertical currents required to produce a sensible 
effect on the magnetic needle. But in the first place we 
may estimate the intensity of the displacement current, 
consistent with electrostatic observations. The numbers 
given in the following quotation from Sir Wm. Thomson^' 
may serve as a basis : “ Even in fair weather, the intensity 
of the electric force in the air near the earth’s surface is 
perpetually fluctuating. The speaker had often observed it, 
especially during calms or very light breezes from the east, 
varying from 40 Daniels elements per foot to three or four 
times that amount during a few minutes.” 
We may then take '01 volt per centimeter in a second as 
a variation not unfrequently occurring. Deduced to C.G.S. 
* Reprints of Papers on Electrostatics and Magnetism, XVI., page 219. 
