1825.] Philosophical Transactions for 1825, Part I. 453 



electricity on the one hand, and of the various investigations in 

 pure magnetism carried on by Prof. Hansteen and Mr. Barlow 

 on the other ; and the paper now before us, another important 

 contribution to our knowledge of this science, will still further 

 tend to establish hie reputation as a natural philosopher. — It 

 commences as follows : — 



" In the paper on the diurnal deviations of the horizontal 

 needle when under the influence of magnets, which the Presi- 

 dent did me the honour to present, I stated that these deviations 

 were partly the effects of changes that took place in the temper- 

 ature of the magnets ; and that although the conclusions which 

 I drew from the observations respecting the increase and decrease 

 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 precise 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 

 induced 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 1 have been 

 obliged to enter, the accomphshment 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. 



" 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 observa- 

 tions on the diurnal changes in the direction of the needle, when 

 under their influence ; to apply the results which I obtained to 

 the correction of the observations themselves, thereby account- 

 ing 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 

 observation . the portion of the arc of deviation due to the 

 changes which he noticed in the temperature of the magnets, . 

 Mr. Christie 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 exert 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 

 towards it, and the other from it, by forces varying inversely as 

 the squares of their respective distances from that pole. 



