AND METEOROLOGICAL INSTRUMENTS, BY PHOTOGRAPHY. 
91 
servations*; and as it is impossible to foresee what points of scientific interest in the 
investigation of magnetic changes may depend on the determination of small quan- 
tities, and on the relation of magnetic variation in warm and cold climates, it may 
be considered not undesirable to ascertain, with the greatest attainable degree of 
accuracy, the temperature coefficients of all magnets to be employed in observing 
the changes of force. In the method here proposed, the magnet is under circum- 
stances precisely similar to those which would exist when it is subsequently used in 
observation ; it may therefore be considered less open to objection than the ordi- 
nary method of deflection, by which the temperature coefficient is inferred from the 
mutual action of two magnets and the earth on each other. Amongst other sources 
of uncertainty in the latter method, may be mentioned the unexplained difference in 
the result that has frequently been observed, according as the marked or unmarked 
pole of the deflecting, has been turned towards the deflected magnet ; arising pro- 
bably from the unequal, and possibly variable, distribution of magnetism throughout 
the bar ; which conditions, if they really exist, will have precisely the same effect on 
the indications of the magnet when under trial, as they would have when it is in 
actual use. 
By introducing the differences only of the scale-readings into the calculation, 
large numerical quantities are avoided, as well as the necessity of adopting a zero 
point, or scale-reading corresponding to no deflecting force. 
* The fact of the decrease of magnetic intensity not being in the simple ratio of the increase of temperature, 
but in some higher ratio, was it is believed first announced by Professor Christie, Sec. R.S., in the Philoso- 
phical Transactions, 1825, p. 63. 
Keppel Street, 1849, June 21. 
