the intensity of magnetic forces, ^c. 6 i 
>vhat might be deduced from a series of such observations as 
I have detailed, rather than to compare the results deduced 
from them with 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 I 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 0.123 M; or taking the intensity of the magnets 1, for 
each degree of increase in temperature we 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 precise effect of change of temperature 
having been previously ascertained, to correct the observations 
according to the difference of the temperatures at which they 
were made. I am not aware that any one has yet attempted 
to make such a correction ; but it is manifest from the expe- 
riments I have described, that it is indispensible, 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 dif- 
ferent, that these temperatures should be registered, and the 
times of vibration reduced to a standard of temperature. 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 
