64- Mr. Christie on the effects of temperature on 
• 
increase of temperature should be the reverse of that pro- 
duced on a magnet, is, I think, a strong argument against 
the hypothesis, that the action of iron upon the needle arises 
from the polarity which is communicated to it from the earth. 
It may be objected to the method which I have adopted 
for determining the diurnal changes in the terrestrial mag- 
netic intensity, that, after the observations have been made, 
they require a correction for temperature, which can only be 
determined by experiments previously made on the magnets 
and needle employed. The same objection may, however, 
be made against the method of determining the intensity by 
the vibrations of a needle. As such a correction has not in 
the latter case been hitherto applied, the results which have 
been obtained relative either to the diurnal changes of in- 
tensity, or the intensities in different parts of the earth, by 
means of observations on the vibrations of a needle, will be 
•SO far incorrect as the needle may happen to have been 
affected by differences in the temperature. The method I 
have described, however, possesses advantages over the 
other : a very considerable one is, that whatever effects are 
produced may easily be observed with considerable precision, 
the time required for each observation being not more than 
five minutes ; another is, that, the magnets being immersed 
in water, as far as regards them, we may command the tem- 
perature at which the observations are to be made, and thus 
limit the correction for temperature to a very small quantity ; 
and it possesses another decided advantage, that whatever 
are the effects produced on the needle by atmospheric 
changes, they are, by means of it, rendered immediately 
visible, and can be observed as they occur. 
