294 ON EARTH-CURRENTS. 



20. The foregoing are the general features of the diurnal varia- 

 tion of the earth-currents, as inferred from the changes of direction 

 and intensity of the horizontal magnetic force. But while the 

 phenomena have much in common, there is at the same time great 

 diversity in the details. This will be evident upon inspection of 

 the diagrams of Plate II., which represent the diurnal changes 

 of the currents at the different stations. The most remarkable 

 peculiarities are those presented at Munich and Prague in Europe, 

 at Catherinburg in Siberia, at Philadelphia in the United States, 

 and at St. Helena and the Cape of Good Hope in the southern 

 hemisphere.* 



21. The great diversity which, in the midst of order, we have 

 thus seen to prevail in the diurnal changes of the earth-currents,, 

 cannot be wondered at, when we consider the endless variety which 

 exists in the distribution of land and water on the earth's surface, 

 as well as in the configuration of the land itself, and in the 

 materials of which it is composed; for all these circumstances 

 affect, in a material degree, the conductibility of the superficial 

 strata. In some of the instances above referred to, we have found 

 a probable connexion between these physical circumstances and 

 the observed facts. We are thus encouraged to hope, that the 

 complex phenomena of the diurnal change may at some future 

 time be completely unravelled, and the peculiar features which it 

 presents at each place traced to their causes. Meanwhile, it would 

 be of great importance to determine more precisely the influence of 

 lines of coast, of mountain chains, and of other geographical and 

 physical conditions, by short series of observations of the diurnal 

 changes of the two magnetic elements at well-selected stations. 

 Such observations would add little to the labour or expense of our 

 numerous exploring expeditions, while they would further in an 

 important degree the knowledge of Terrestrial Magnetism. 



* The remarks in the original Paper upon these diversities, and upon their prohable 

 causes, have been here omitted. 



