ON BRITISH, COLONIAL MAGNETIC OBSERVATORIES. 3G5 
similarvariation was discernible in other terrestrial magneticvariations,which, 
like the disturbances, were regulated by solar hours ; and I had soou the gratifi¬ 
cation of perceiving that the mean diurnal variation in each year (that of 
which I spoke in the early part of this exposition) exhibited a variation in 
its amount in different years, which, like the disturbances and the solar 
spots, had a decennial period; that the epochs of minimum ami maximum 
were also 1843 and 1848; and that in the two broken half-periods, the one 
previous to 1843 and the other subsequent to 1848, thu same law of alternate 
progressive increase and decrease was manifested. In the conclusion of 
which I am now speaking, viz. the decennial fluctuation in the magnitude of 
the diurnal variation, I was, without being aware of it, preceded (in one of 
the magnetic elements, viz. in the declination) by our associate, M. Lamont, 
who had perceived it in the examination of the observations of the same 
years made in the Magnetic Observatory at Munich, but to whom it is pro¬ 
bable that the account, of M. Schwabe's observations of the periodicity of 
the solar spots was not then known, for otherwise 1 have little doubt that the 
coincidence would have attracted his attention. 
From the Munich observations, the years 1843 and 1818 were also found 
to be the epochs of minimum and maximum in the magnitude of the diurnal 
variation of the declination dependent on the solar hours. 1 have since 
ascertained that these are also the years of maximum and minimum of a 
periodical fluctuation in the diurnal variation of the inclination of the mag¬ 
netic needle and of the intensity of the magnetic force of the earth, as well 
as of the horizontal direction of the needle, at all the four stations at which 
the Colonial Observatories were established. 
It is fully admitted, that before so important a fact as that of a correspond¬ 
ence in period and epoch between affections of the luminous disc or photo¬ 
sphere of the sun, and magnetic variations observed upon the earth, can 
he regarded as positively established, it will require to be confirmed by ob¬ 
servation of the recurrence of the coincidence in several successive periods. 
But so far as the limited duration of the Colonial Observatories has permitted 
fho comparison to be made, the coincidence thus brought by their means 
into public view, and submitted for a further examination, (which it cannot 
•fill to receive,) has been manifested alike by every one. of the magnetic varia¬ 
tions, which, by their dejiendcnce on solar hours, are traceable to the sun as 
their primary cause . 
lad the observatories borne no other fruit of an analogous description but 
,a '''Inch I have thus brought before you regarding the Sun, they would 
*®ve been entitled to the distinction of having at least indicated a coincidence 
e ween terrestrial magnetic variations and the phenomena of one of the 
eavenly bodies, which coincidence is characterized by the absence of any 
^jscoverable corresponding variations of temperature-, or terrestrial affections 
-"^.through whose agency or intermediation the magnetic changes 
In th K * l >ro ^ UCl ^' toe evidcnccin regard to the sun docs not stand alone, 
j e c ? s< - °1 toe Moon also, we have been able to trace unquestionable evi- 
den i ex ' 9ter *cc of a magnetic variation at the surface of the earth, 
t,!,} 1 J ut , on a c ' l »nge in the moon's place relatively to the meridian of the 
Piiatelv 0)Ser j at ' 0,) at toe different hours of the day, and therefore appro- 
rem r ,? ain . l *‘ e ^ unar diurnal magnetic variation. I need scarcely 
of temriA ^ m l * le < r ast; nioon > any supposition of the intermediation 
would IV* a - S , , I )ro< lucing cause of the terrestrial magnetic affection 
to infpi. 1 F . lo » evcu niore improbable than we appear to have reason 
9 mter it to be m the case of the solar spots. 
