358 
REPORT -1854. 
cognize the sun as its primary cause, not only from the circumstance that its 
cycle or period coincides with that of a solar day, but also from the fact that 
the same changes of direction are found to occur at different places on the 
globe situated under different meridians, at the same (or very nearly the 
same) hours of solar time at each of those places. Thus, for example, in 
England, the north or marked end of the needle points to the extreme west 
of its daily movement when tho sun is about two hours past the meridian, 
and the same thing is found to take place at the same hour of local time over 
the whole of Europe, although, as you are all aware, the absolute time cor. 
responding to the same solar hour in the eastern and western parts of 
Europe is very different. The diurnal variation having thus its character 
dependent on and determined by the solar hour , we have this warrant for 
regarding the sun as its primary or original cause. 
It has also long been known as a general fact that in Europe the amount 
of the diurnal variation is greater in summer than in winter, a fact wlueu 
found a ready explanation in the prevailing opinion which ascribed the mag¬ 
netic variations to thermo-electric causes, or to some other less definite 
modes of operation of variations of temperature, and which were naturally 
supposed to act with greater intensity in summer than In winter, inasmuc 
as the diurnal variations of temperature are greatest in summer; yet thea* 
were not wanting, even in the last century,observers in England who remand 
that the increase of the diurnal variation in summer by no means comci t 
with, but considerably preceded, the period of the greatest summer heihW 
also the period when the variation oi' teniperaturo in the course of the flay 
was greatest. Hut their remarks appear to have hud but little weight in op 
position to a favourite hypothesis, and it lias remained for the jar j* 101 
extensive, more varied and more decisive conclusions derived from the o r 
vations of the British Colonial Observatories to generalize the phamomena, J 
manifest their utter irreconcilability with the explanations which wou' 
ascribe the change in the amount of the diurnal variation in different pa 
of the year to local variations of temperature, and to show that the irufl 
planation must rather be sought in some more direct action oi the sun on 
magnetism of the earth. . 
I shall endeavour to make this clear to you by a graphical represc-niai <■ 
which will require a few words of preliminary explanation. 1 he part o ^ 
general phmnomeiion of the diurnal variation to which I desire s]ieei»l? 
dnect your attention, is its alteration in character and amount w ui *■ ' 
parts of the year and in differeut pnrta of the globe. This part of the y 
nomenon admits of being viewed separately ; the mean diurnal variation 
the year, the mean in fact of all the months, constituting, as it were, an 
or a zero-line, in comparison with which the difference which takes pW# 
the diurnal variation in the different mouths may be shown, and the alien»■ 
may thus bo brought to bear more immediately and more exclusively, 
what may be not inappropriately termed the annual variation of the 
i lt raa f . b ,° s,,own with the greatest simplicity for any!#* 11 " 
T.i for every month of the year thedilfej^ 
uml the mn ketwefln the mean diurnal variation of the w ho « . 
the alterZn d, !Pf 1 Variation » that particular month; and by comp** 
vors ihEiv , ,Ch . y° fi,,d tn ^ P^ce in tl.e different months a ^ 
<R.Kted y wfth a ? te ? T‘ eacb otl,er on the surface of the earth, and iWJPj 
teinneraturp VI ^A eac l lcr 1,1 regard to all those physical chann*^ 
nositfon "L h ,’ Ch arc dependent on and vary with 
!ions are dL n t0 test decisively the question whether those ^ 
° local circumstances, or are the result of some general 
