84 The Rev. II. Lloyd on the Results of Observations made at the 



the greatest easterly elongation being then earlier than 8 a. m. It is manifest, 

 however, that such an explanation will not apply to the result deduced, as in the 

 present instance, from the diurnal curve ; and there can be no longer any doubt 

 of the reality of the phenomenon. 



12. The physical dependence of the changes of declination upon the sun is 

 evident from the fact that they observe a diurnal and an annual period. The 

 conclusion deducible from this fact has been confirmed by the leading features 

 of the diurnal movement. Thus it has been long ago observed, that the time 

 of greatest westerly elongation follows the sun's meridian passage at a nearly 

 constant interval ; and that the times of greatest easterly elongation, in the 

 morning and evening, are in Uke manner connected, although not so closely, 

 with the hours of sunrise and sunset. The greater magnitude of the range, in 

 summer than in winter, is another obvious confirmation of the same view. 



We may, I believe, disregard, as wholly untenable, the hypothesis originally 

 proposed by Coulomb, in which the influence of the sun is assumed to be direct, 

 and the effect of magnetic polarity in that body. It is easy to show that, if such 

 an action exist at all, it cannot certainly account for the principal part of the 

 observed effect. But, without dwelling on the negative side of the question, I 

 hope to show that the sun acts indirectly, by means of his heating power exerted 

 upon the earth's surface. This has been assumed by Canton, and since by 

 Professor Christie, in the hypotheses which they have severally devised to 

 account for the diurnal variation of the declination ; but the evidence upon 

 which it rested did not extend beyond the facts which have just been stated. 

 It will appear from the following examination, that the connexion between the 

 changes of declination and those of temperature is more intimate than has been 

 hitherto supposed. 



13. The force which produces the deviation of the magnet from its mean 

 position, at any moment of the day, is measured by the sine of the deviation, — 

 or, since the deviation is small, by the angle of deviation itself, or by the ordi- 

 nate of the diurnal curve ; and the sum of all these forces throughout the day, or 

 the integral of the diurnal action, is measured by the area of the diurnal curve. 

 If, then, the diurnal variation of the declination be the result of the diurnal 

 variation of temperatm-e, we should expect to find a marked correspondence 

 between the areas of the diurnal curves of the two elements,thi'oughout the year. 



