304 ON THE PROBABLE CAUSES 



and structure of the earth's surface which influence the direction 

 and magnitude of the currents ; but I incline to think that the 

 principal one is that ahove stated, viz. the distribution of land and 

 water in the vicinity of the place of observation. It may be, also, 

 that this cause is sufficient to account for some of the peculiarities 

 in the form of the diurnal curve noticed in my former communica- 

 tion, and there referred to other causes. Thus, it is not improbable 

 that the persistent direction of the current at Munich, there referred 

 to the influence of a mountain range, may be, in fact, the result of 

 the proximity of the Adriatic Gulf, which lies nearly in the direc- 

 tion of the persistent current. 



In the preceding remarks I have referred only to the regular 

 diurnal changes. I believe that the irregular are produced by the 

 same forces, but operating in a somewhat different manner. The 

 regular currents are produced, as I conceive, chiefly by the separa- 

 tion of the two electricities by evaporation, under the action of the 

 sun ; while the disturbance-currents are caused by their rapid re- 

 combination, through the medium of moisture, in the lower strata 

 of the atmosphere.* In connexion with this view, I may refer to 

 the fact which has been established by an examination of the mean 

 effects of the magnetic disturbances,! namely, that the epochs of the 

 maxima of the disturbance-currents depend, in their mean values, 

 upon the sun's hour-angle, and are independent of the longitude 

 of the place. This result is in accordance with the hypothesis 

 which ascribes these currents to changes in the sun's calorific 

 agency, and to the meteorological effects which these engender. 



In the limits within which it is necessary to confine this 

 abstract, I have been able only to refer to some of the leading 

 facts in confirmation of the hypothesis which I have ventured to 

 propose ; and I am obliged to omit altogether all reference to the 

 objections which will probably be raised against it. There is, how- 

 ever, one fact which appears at first sight to offer a formidable 

 difficulty to its reception, and which it seems necessary to notice 

 here. The regular magnetic changes are greater in summer than 



* This hypothesis as to the cause of magnetic disturbances is due to M. de la Rive ; 

 but his views ^ respecting the laws of the resulting currents are, as I have elsewhere 

 shown, mconsistent with the phenomena. The regular diurnal changes of terrestrial 

 magnetism are ascribed by M. de la Rive to a direct electrical action emanating from 

 the sun. 



t Prceeedings of the Royal Irish Academy, April 28, 1862. 



