24 ^ 
of nature, as embodying all the pertinent facts that obser- 
vation has acquainted us with. 
In speaking of attempts of the earlier magneticians to 
explain the distribution of magnetic force on the earth’s 
surface by hypotheses — first of a central magnet, then of a 
magnet displaced from the centre, and then of two eccentric 
magnets. Gauss says, ‘^It will be best to abandon entirely 
this mode of proceeding, which reminds one involuntarily 
of the attempts to explain the planetary motions by con- 
tinued accumulation of epicycles.” What would have been 
his astonishment, then, if he could have foreseen that nearly 
]ialf a century after the publication of his “ General Theory 
of Terrestrial Magnetism,” his investigation had met with 
so little appreciation, that even a disciple professing to follow 
in his footsteps could still resort to crude hypothesis where 
the master had shown the way to knowledge; that time had 
only substituted hypothesis as to which one or two subordi- 
nate terms of the expression of a potential should be ten- 
tatively adopted as the whole, in the place of hypothesis as 
to one or two magnets fixed within the earth ; and that on 
questions of importance, conclusions legitimately deducible 
only from the evidence of the full potential had been drawn 
from the restricted potential of such hypothesis. 
Yet all this has happened, as I will proceed to show by 
an examination of a paper ‘‘On the Diurnal Period of 
Terrestrial Magnetism,” by Arthur Schuster, F.RS.,* read 
before the Manchester Literary and Philosophical Society, 
on the 20th January, 1886. This paper is occupied, mainly, 
by an endeavour to prove, by the use of the forms of the 
harmonic analysis, that the seat of the forces concerned in 
the production of the diurnal variations lies outside the 
earth ; and the author claims that his results, as far a,s they 
go, give an emphatic answer in favour of the supposition, 
^ Pi-oceediugs— Literary and Philosophical Society} Vol. XXV,, NOi 7, Session 1885-0» 
