92 The Rev. H. Lloyd on the Results of Observations made at the 



changes of temperature of the earth's crust ; for it is known that the epochs of 

 mean temperature, as well as those of the maximum and minimum temperature 

 of the soil, are retarded, and follow the corresponding epochs for the tempera- 

 ture of the air by an interval which is proportional to the depth. 



These retardations, when observation shall have determined them with 

 greater precision, will probably be found (in accordance with the results of 

 Professor Forbes's experiments) to be different in different localities, depending 

 upon the conductibility of the soil. 



21. It remains to notice the bearing of the remarkable relations between 

 the annual and the secular changes, stated in Art. 18. It would seem to follow 

 from these relations, that the two classes of changes are physically connected ; 

 and therefore that the secular, as well as the annual variation, is due to the 

 heating power of the sun exerted upon the earth's crust, — altliough not only 

 the magnitude, but even the direction of the change is different at different 

 times. It is not easy to frame even a conjecture as to the nature of such an 

 agency, in the case of the secular change. 



Disturbances. 



22. Having examined the periodical and the secular variations of the decli- 

 nation, it remains to consider those which, from our ignorance of their laws, 

 we have been accustomed to call " ii-regidar." 



Professor Keeil seems to have been the first to notify the remarkable fact, 

 that magnetic disturbances occur more frequently at certain hours than at 

 others ; and, that the direction, as well as the frequency of these movements, 

 has a dependence upon the time of the day. Colonel Sabine has since made 

 a more complete and elaborate examination of this question, in the discussion 

 of the Toronto observations, and has arrived at conclusions for the most part 

 confirmatory of those obtained by Professor Kreil. 



In these investigations, however, those disturbances only are taken into 

 account which exceed a certain arbitrary limit ; and of these the frequency 

 is considered without any reference to their magjiitude. In examining the 

 question of the periodicity of disturbances, I have thought it advisable to pur- 

 sue a different course. I have taken the differences between each result, and 

 tlie monthly mean corresponding to the same hour, and combined these diffe- 



