298 Royal Society :-— 
phenomena, whether it appear in the form of momentary, daily, 
monthly, or annual change and restoration ; or in progressive changes 
not compensated by counter-changes, but going on continually accu- 
mulating in one direction, so as in the course of many years to alter 
the mean amount of the quantities observed.’””—Report, pp. 1, 2. 
With reference to the first of these two branches, viz. the actual 
distribution of the magnetic influence over the globe at the present 
epoch, the Report goes on to state :—‘ The three elements, viz. the 
horizontal direction, the dip, and the intensity of the magnetic force, 
require to be precisely ascertained, before the magnetic state of any 
given station on the globe can be said to be fully determined .. . . 
and as all these elements are at each point now ascertained to be in 
a constant state of fluctuation, and affected by transient and irregular 
changes, the investigation of the laws, extent, and mutual relations 
of these changes is now become essential to the successful prosecu- 
tion of magnetic discovery.” 
With reference to the second branch, viz. the secular and periodical 
variations, it is observed that—< The progressive and periodical 
being mixed up with the transitory changes, it is impossible to sepa- 
rate them so as to obtain a correct knowledge and analysis of the 
former, without taking express account of and eliminating the latter ;” 
and with reference to the secular changes in particular, it is re- 
marked— These cannot be concluded from comparatively short 
series of observations without giving to those observations extreme 
nicety, so as to determine with perfect. precision the mean state of the 
elements at the two extremes of the period embraced ; which, as 
already observed, presupposes a knowledge of the casual deviations.” 
It is clear from these extracts that in the discussion of the obser- 
vations the first point, in the order of time, ought necessarily to be 
an investigation into “ the laws, extent, and mutual relations of the 
transient and,” (as they were called at the time the Report was 
written,) ‘irregular changes,” as a preliminary step to the elimi- 
nation of their influence on the observations from which a correct 
knowledge and analysis of the progressive and periodical changes were 
to be obtained. It will be proper to show therefore, in the first 
place, what the Observatories have accomplished in regard to the so- 
called casual or transitory variations. 
Casual Variations.—All that was known regarding these phe- 
nomena at the period when the Report of the Committee of Physics 
was written, was, that there occurred occasionally, and, as it was 
supposed, irregularly, disturbances in the horizontal direction of the 
needle, which were known to prevail, with an accord which it was 
impossible to ascribe to accident, simultaneously over considerable 
spaces of the earth’s surface, and were believed to be in some un- 
known manner connected, either as cause or effect, with the ap- 
pearances of the aurora borealis. The chief feature by which the 
presence of a disturbance of this class could be recognized at any 
instant of observation,—or by which its existence might be sub- 
sequently inferred independently of concert or comparison with other 
Observatories, appeared to be, the deflection of the needle from 
