of the Terrestrial Magnetic Intensity. 1 47 
those obtained by others, for which purpose it would have been 
necessary to have continued them for a greater length of time. 
“ We have seen, that with the magnets X made use of, their 
intensity being nearly 218 M, at the temperature 60°, a change 
in their temperature of 1° would cause a change of intensity of 
0.128 M ; or taking the intensity of the magnets 1, for each 
degree of increase in temperature w r e should have a decrease of 
intensity of 0.000564. Now, if the same, or nearly the same, 
take place with all magnets, it is evidently necessary, in all 
cases where the terrestrial magnetic intensity is to be deduced 
from the vibrations of a needle, that great care should be taken 
to make the observations at the same temperature ; or, the pre- 
cise effect of change of temperature having been previously as- 
certained, to correct the observations according to the diffe- 
rence of the temperatures at which they were made. X am not 
aware that any one has yet attempted to make such a correction ; 
but it is manifest from the experiments I have described, that it 
is indispensable, in order to deduce correct results from the times 
of vibration of a needle in different parts of the earth, where the 
temperatures at which the observations are made are almost 
necessarily different, that these temperatures should be regis- 
tered, and the times of vibration reduced to a standard of tem- 
perature. It appears to me, that the effects will be the most 
sensible in large and powerful needles ; and consequently, in 
making use of such, the reduction for a variation of temperature 
will be most necessary. There would be no difficulty in this 
reduction, if we could give, in terms of the intensity of any mag- 
net, the increment or decrement of intensity corresponding to a 
certain decrement or increment of temperature at all tempera- 
tures. To determine this accurately would, however, require a 
great variety of experiments to be made with magnets of very 
different intensities ; but, as I have not made these, X must con- 
tent myself for the present with pointing out some of the facts 
which I have ascertained from more extended experiments than 
those X have already given, reserving the detail of these experi- 
ments for another opportunity, should they be deemed of suffi- 
cient interest. 
64 These experiments were made with a balance of torsion, 
K % 
