27 
writer of the paper under discussion, let us take, as a stan- 
dard of comparison, the full surface potential of the first 
and second degrees, the unknown coefficients of which have 
to be determined in accordance with Gauss’ own mode of 
procedure. 
The writer’s first step, and one the consequences of which 
run all through his investigations, is to assume that — 
~dQ d\ ’ Is d\ ' ^ 
and it is premised by the remark, “ Observation tells us, 
that all over a given circle of latitude we may take the 
variation to be very nearly the same for a given local time.” 
The correctness of the assumptions must be estimated, of 
course, by the exactness of the statement on which they are 
based ; and it must be admitted that, except in high lati- 
tudes, the variations do, in a rough manner, conform to the 
rule laid down ; but it is known that in proceeding round 
the north magnetic pole, it is the magnetic meridian of the 
place and local time to which the variations of declination 
are referred, when it is said that there is similarity in the 
type of variation ; and as such magnetic meridians have all 
inclinations (from 0° to 360°) to the respective astronomical 
meridians, it is obvious that the rule is far from correspond- 
ing to the facts in that region, as far, indeed, as it possibly 
could be in some parts of it. It is to be observed, too, that 
in another place“[* the writer has inferred from the variations 
of this region, that there a vertical component of an electric 
current should be found to cross the earth’s surface : an 
inference antagonistic to the present assumption of the 
existence of a potential, unless the vertical current be fur- 
ther assumed to be of constant intensity. I cannot, thus, 
regard these assumptions as a sound basis on which to build 
_ * Page 123 of the paper where the nomenclature is the same as above, except that 
t IS used for 9 , and X, Y for X^ , Y^. 
t Report of British Association, 1885 ; and Proceedings —Literary and Philo- 
sophical Society, Vol. XXV., No. 7, pp. 115 and 122. 
