"معاملات الجمعية الجغرافية في بومباي. من يناير ١٨٦٣ حتى ديسمبر ١٨٦٤. (تحرير السكرتير.) المجلد XVII." [‎١‎٩‎٣] (٥٢٤/٣٦٤)

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Art. X.— On an effect of the Atmosphere upon the Magnetic Intensity
of the Earthy supposed to he found in the Bombay Magnetic
Observations, from 1847 to 1862.—By John xAllan Broun,
Esq., F.R.S,
[Read before the Society, January 21st, 1864.]
The conclusion that such an effect existed, appeared first in the
Bombay Observations for the year 1847 (p. 479), where a reference is
made to the introduction to the volume for the evidence ; this, however,
I have found only in the introduction to the volume for 1848 (pp.
xxii, xxiii, and xxiv). Corrections and discussions have been based
on this result in fifteen successive volumes, in every one of which it
has been re-asserted. If true, it would be a fact of very great import
ance, and the grounds upon which the discovery is based merit careful
consideration.
The conclusion is thus stated by Captain Montriou in the volume
for 1847 (p- 472) -“To cause the above observed effect, it was sup
posed that the air surrounding the Magnetic Observatory was mag
netic, and that its moment altered with the changes of its temperature ;
consequently, this air affecting the needle of the Horizontal Force Mag
netometer, in addition to the magnetic force of the earth, caused the
observed phenomenon. The magnetism in air may have been induced by
the earth ; but whatever be the cause, the effect of the air was certainly
felt, and it was necessary that it should be removed (considering it as
a local cause) to obtain the true changes of the earth’s magnetism.”
The following are the grounds upon which the result appears to
depend :—Three tables have been formed: in the first the monthly
mean values for 1847 of the bifilar scale readings, corrected for the
effect of temperature upon the magnet as shown by a thermometer in
the same box is compared with the monthly mean temperature shown
by the box (or attached) thermometer, and by a thermometer placed
three feet above the ground outside the Observatory ; in the second
table similar comparisons are made of the even hourly means for 1847 ;
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