189 



In the preceding remarks I have referred only to the regular dinrnal 

 changes. I believe that the irregular are produced by the same forces, but 

 operating in a somewhat different manner. The regular currents are pro- 

 duced, as I conceive, chiefly by the separation of the two electricities by 

 evaporation, under the action of the sun ; while the disturbance- currents 

 are caused by their rapid recombination, through the medium of mois- 

 ture, in the lower strata of the atmosphere.^' In connexion with this 

 view, I will, for the present, merely refer to the fact which has been es- 

 tablished by an examination of the mean effects of the magnetic distur- 

 bances (Proceedings, April 28, 1862) — 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 meteorolo- 

 gical 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 confirma- 

 tion of the hypothesis which I have ventured to propose ; and I am 

 obliged to omit altogether all reference to the objections which will pro- 

 bably be raised against it. There is, however, one fact which appears at 

 first sight to offer a formidable difficulty to its reception, and which it may 

 be necessary to notice here. The regular magnetic changes are greater 

 in summer than in winter ; while with the electrical tension, and its 

 changes, it is the reverse. This objection, however, disappears when it 

 is viewed more closely. The physical quantity measured by our elec- 

 trometers is not the absolute electric tension, but its variation with the 

 height ; while the electric changes which engender terrestrial currents 

 are the variations as depending on horizontal distance. It is easily cour 

 ceivable that these should not correspond. In fact, it is natural to sup- 

 pose that in summer the zero -plane, which separates the two electricities, 

 should rise considerably ; and thus that the variations for a given increase 

 of altitude (which probably diminish with the distance from that plane) 

 should lessen, although the absolute tensions, as well as the changes in 

 horizontal distance, may be greater. 



It would be of importance, in reference to this inquiry, to institute 

 electrical observations of a totally different kind from any which we 

 now possess, and to measure the differences of tension as depending on 

 horizontal distance. There seems to be no difiiculty in the way of such 

 observations, — at least none greater than those which present themselves 

 in the ordinary observations of atmospheric electricity ; and the results 

 would probably do more to clear up the physical aspect of these complex 

 and interwoven phenomena than any other observational means. 



* 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, 

 inconsistent with the phenomena. The regular diurnal changes of terresti-ial magnetism 

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



E. I, A. PEOC. — VOL. vm. 2 C 



