on the Intensity of Magnetic Forces , tyc. 141 
I drew from the observations respecting the increase and de- 
crease of the terrestrial magnetic forces during the day would 
not be materially affected, it was my intention to undertake a 
series of experiments for the purpose of determining the pre- 
cise effects of changes of temperature in the magnets, so as to be 
able to free the observations entirely from such effects. 
“ These experiments were immediately made ; but I was in- 
duced, from some effects which I observed, to carry them to a 
greater extent, in the scale of temperature, than was necessary 
for the object which I had at first in view. In consequence of 
this, and the length of the calculations into which I have been 
obliged to enter, the accomplishment of my purpose was delayed 
for a considerable time, and continued indisposition has since 
prevented me, until now, completing the arrangement of the 
tables of results. 
44 In the present paper, I propose to detail the experiments 
which I made, in order to determine the effect of changes of 
temperature on the forces of the magnets, to the extent to 
which I observed their temperature to vary, during my obser- 
vations on the diurnal changes in the direction of the needle, 
when under their influence ; to apply the results which I ob- 
tained to the correction of the observations themselves, thereby 
accounting, for the apparent anomalies noticed by Mr Barlow 
and myself, in the observations made in-doors and in the open 
air; and, by means of these corrected observations, to point 
out the diurnal variations in the terrestrial magnetic intensity.*” 
Having found it impracticable to determine purely from ob- 
servation the portion of the arc of deviation due to the changes 
which he noticed in the temperature of the magnets, Mr Chris- 
tie was, therefore, under the necessity of having recourse to 
theory ; and he adopted the simplest, and that which is most 
generally received, viz. that the forces which two magnets ex- 
ert upon one another may be referred to two centres or poles 
in each, near their respective ends ; and that for either pole in 
one of the magnets, one pole of the other magnet is urged to- 
wards it, and the other from it, by forces varying inversely as 
the squares of their respective distances from that pole. 
After this statement, he proceeds to explain and exemplify 
the application of the theory to the investigation detailed in the 
