'The Transactions of the Bombay Geographical Society. From January 1863 to December 1864. (Edited by the Secretary.) Volume XVII.' [‎194] (365/524)

«
194 EFFECT OF THE ATMOSPHERE UPON THE
in the third the quantities compared are the monthly mean diurnal
ranges*
An inspection ot all three tables shows, that when the temperature
increases the bifilar readings also increase. As the monthly mean
temperature within the box 'varies neaily as much as the monthly
mean temperature outside the Obseivatory three feet above the ground
the comparison indicated gives nearly the same number of divisions of
the bifilar scale for a change of 1° Fahr. in both instances ; or
increases of 1° Fahr. within the box, and outside the Observatory
are respectively equivalent to increases of 1 * 17 and 0'91-ma/J scale
divisions of the bifilar reading. These values give no information as to
which temperature they are due, and an appeal is made to the second
table, where the two hourly variations of temperature within the box
are only about half of those outside the Observatory : this table seems
To show that, for increases of 1° Fahr. the variations of bifilar reading
are 278 and l-19 small scale divisions respectively ; and a further refer
ence to the third table, where a similar difference exists for the tem
perature variations, gives 3*01 and 1*55 scale divisions respectively
It was concluded :—“ From this it is evident that the near agree
ment of the values under the head of circumambient air (7. e. external
temperature) and the wide difference of the values under the head of
attached thermometer, sufficiently proves that the effects of tempera
ture on the horizontal intensity are due only to the circumambient
air.”— Introduction 1848, p. xxiv.
I think that there is no foundation for this conclusion, and that it is
easy, from the data in the Bombay Observations themselves, to explain
these co-efficients, without any hypothesis as to the circumambient air.
Ib .3 a well known fact, that the position of the bifilar magnet depends
upon i - » temperature in such a way, that when the temperature increas*
es the scale readings decrease or seem to show a decrease of the earth’s
magnetic force. In the tables employed by Captain Montriou for this
discussion, a correction Aac/been already applied on this account; the
t correction employed by him being 3*2 small scale divisions for 1°
these are the quantities properly deductible from the three tables (when some
sma U eiTor3 are corrected), but they differ slightly from those obtained bv Captain
Montriou.