398 
MR. CHRISTIE ON THE MAGNETICAL OBSERVATIONS 
I shall not now make any remarks on the results contained in the foregoing Table, 
but proceed to describe the method I adopted for determining the correction neces- 
sary to be applied, in order to reduce the measures of intensity at different tempe- 
ratures, to be derived from the times of vibration in this and the preceding Table, to 
measures of intensity at the same temperature. 
Determination of the Correction for the Difference of Temperature. 
Since I first pointed out the necessity of applying a correction for the difference in 
the temperature at which observations for determining the magnetic intensity may 
have been made *, such a correction has, in many instances, been applied ; but its 
amount has seldom been determined from direct experiments on the individual needle 
employed in the observations from which the intensities were to be deduced. This I 
consider to be essential ; for in the numerous experiments which I have made on the 
subject, I have found that, in different needles, there is a very considerable difference 
in the amount of this correction ; and M. Kupffer’s experiments -f- show that this 
amount depends upon the nature and the temper of the steel employed. I therefore 
proposed to determine this correction for each of the needles which had been em- 
ployed by Captain Back. For this purpose I employed an eartheiiware vessel having 
a wooden bottom firmly fixed in it, to which the needle under trial could be securely 
attached by means of ivory pegs. Above this vessel was placed a stage, supporting a 
compass having a small trial-needle, T36 inch long, delicately suspended by a single 
fibre of silk. The time in which this needle performed 100 vibrations having been 
carefully determined by two trials, the needle under examination was fixed to the 
wooden bottom of the vessel below, but separated from the wood by narrow slips of 
glass. This needle was placed with its centre exactly below the centre of the trial- 
needle, its axis in the magnetic meridian and its marked end towards north ; and 
the distance of the upper or vibrating needle from the lower was so adjusted that its 
marked end still pointed north, but with a greatly diminished force. A small ther- 
mometer was placed on the glass of the compass, to indicate any change of tempera- 
ture that the vibrating needle might undergo ; and another thermometer was placed 
in the vessel containing the needle under trial, with its bulb close to that needle, and 
not touching the bottom of the vessel. Water, of as low a temperature as could be 
obtained, without employing chemical means, was then poured into the vessel con- 
taining the needle under trial ; the temperature of this needle and also of the vibrating 
needle having been noted, the time of vibration of the latter was determined by two 
successive trials, and the temperature of both needles was again ascertained. Water 
of a higher temperature was then poured into the vessel, and the whole well agitated ; 
and as soon as the temperature became uniform, the foregoing observations of tem- 
perature and time of vibration were repeated. In this manner, the temperature of the 
needle was successively raised to the highest temperature at which observations had 
* Philosophical Transactions, 1825, p. 61. + Annales de Chimie, tom. xxx. p. 113. 
