442 M. Haichnger on the Original Formation of Aerolites 



the absence of all currents, is indicated by the instrument itself; 

 the results therefore furnish the measures of the forces by which 

 they are produced. 



The next and most important step in this inquiry will be to 

 assign the physical cause of these phenomena. The existence of 

 electric currents traversing the earth's crust has hitherto been 

 maintained as a hypothesis, on account of its supposed adequacy 

 to explain the terrestrial magnetic changes. Now, however, 

 their existence is proved not only to be a fact, but also a fact 

 sufficient to explain the phenomena. It remains therefore only 

 to ascertain their source; and it will be for those who deny 

 that the sun operates by its heat in producing the phenomena of 

 terrestrial magnetism, to assign to these currents a more pro- 

 bable origin. 



P.S. While these pages were passing through the press, the 

 writer received, by the kindness of Dr. Lamont, a copy of a 

 further communication from him on the same subject, in a letter 

 to Professor De la Rive, dated Oct. 10, 1861. In this letter, Dr. 

 Lamont seems to recede from the view expressed in a former 

 letter (see note, supra), and expresses his belief that the diurnal 

 variations of terrestrial magnetism cannot be explained by the 

 direct action of electric currents propagated on the earth's sur- 

 face ; and he advances the hypothesis, that the regular portion 

 of these variations is due to a peculiar influence of the sun, their 

 irregular fluctuations alone being caused by the earth-currents. 

 These conclusions seem to be irreconcilable with Mr. Barlow's 

 observations, and are opposed to the inferences which I have 

 drawn from them in the preceding pages. 



The scientific public will therefore await with interest the de- 

 tailed publication of Dr. Lamont's investigations on this im- 

 portant subject, in which, it may be hoped, he will give some 

 clue to the explanation of this seeming discordance. 



Trinity College, Dublin, 

 Nov. 16, 1861. 



LVII. Considerations respecting the Original Formation of Aero- 

 lites. — Part II. By W. Haidinger, For. Mem. R.S.L. §E. 

 and Director-General of the Geological Survey of Austria. 

 [Concluded from p. 3^1.] 



IF the phenomena attending the fall of meteorites upon our 

 own earth offer serious difficulties, considerations concern- 

 ing the condition of their previous existence is by far a more 



