horizontal needle when under the influence of magnets. 389 
produce only the same effect at the distance 15.25 inches 
which they previously produced at the distance 1 5-95 ; but 
during the whole of this time they appear to have exerted 
the same, or very nearly the same energy at the same tem- 
perature. It would appear then, that the permanent destruc- 
tion of their power must have arisen from their being heated 
beyond a certain degree, and it does not seem improbable, 
that solar heat may have a greater influence than any other 
in producing such an effect. 
Seeing then that such is the effect of the temperature of 
the magnets on the changes which take place in the points of 
equilibrium, it might perhaps be supposed that variations in 
it, if not the only, were the principal causes of the peculiari- 
ties which I have pointed out. Although I felt persuaded 
that this was not the case, I delayed presenting these obser- 
vations until I had seen such peculiar effects taking place 
independent of the temperature of the magnets. For this 
purpose, I have been observing the changes in the points of 
equilibrium, in a situation where the magnets were exposed 
to very small variations in temperature, the greatest during 
eleven days not having exceeded 8 ° Fahrenheit, and not 4 0 
during the observations of any day. The changes in the 
temperature of the magnets I regularly noted, and expected 
to see, in perhaps some few instances, that the changes in 
the situations of the points of equilibrium would be in oppo- 
sition to those which would arise from the change of tempe- 
rature in the magnets alone. My expectations have been 
more than answered ; since I have noticed this to take place, 
not in a few instances, which, being decided ones, would have 
been sufficient to have established the principle, but re- 
peatedly in the observations of the same day. 
