438 The Rev. Dr. Lloyd on Earth-currents, and their 



the horizontal needle is westerly (i. e. from 8 or 9 a.m. until the 

 evening), and northwards when the variation is easterly (t. e. 

 during the night and early part of the morning), " yet simulta- 

 neous observations showed no similarity in the path described by 

 the magnetic needle and the galvanometer." 



An examination of Mr. Barlow's galvanometric observations 

 led me, some time since, to an opposite conclusion ; and at the 

 last meeting of the British Association I stated my conviction, 

 founded on these observations, that the earth-curi'ents, whose 

 continuous flow Mr. Barlow has the merit of establishing, would 

 eventually explain all the changes of terrestrial magnetism, both 

 periodic and irregular. I now proceed to state the grounds of 

 this conviction, and to show, from Mr. Barlow's observations, 

 that the diurnal changes of the earth-currents correspond with 

 those of the horizontal component of the earth's magnetic force*. 



Let us suppose, then, that the forces which act upon the hori- 

 zontal needle, and which cause it to deviate from its mean posi- 

 tion, are due to electric currents traversing the upper strata of 

 the earth in a horizontal direction ; and let f denote the inten- 

 sity of the current in the magnetic meridian, positive when flow- 

 ing northwards, and vice versa; and 77 the intensity of the cur- 

 rent perpendicular to the magnetic meridian, positive when 

 flowing eastward, and vice versa. Then the force of the current 

 in any direction, making the angle e with the magnetic meridian 

 (measured to the east of north), is 



<£ = £cose + 77 sin e. 



Now £ is proportional to the force which deflects the freely sus- 

 pended horizontal needle from its mean position, or to XA\|r, X 

 being the horizontal component of the earth's magnetic force, 

 and A-v/r the change of declination expressed in parts of radius. 

 Similarly, rj is proportional to the force which deflects from its 

 mean position a magnet which is maintained (by torsion or 



* The first proof of a correspondence between the magnetic variations, 

 and the changes of the earth-currents, seems to be clue to Dr. Lamont of 

 Munich, in a letter dated July 29, 1861, which was read by the Astronomer 

 Royal at the hist meeting of the British Association. Dr. Lamont states 

 that he has found " that electric currents, or (as they may be more pro- 

 perly termed) electric waves, varying in direction or intensity, are constantly 

 passing at the surface of the earth, and that these waves correspond per- 

 fectly with the variations of terrestrial magnetism." The correspondence 

 here referred to seems to relate to the smaller and more rapid variations 

 of the terrestrial magnetic force. But in a letter to Prof. Heiss, dated 

 September 1, Dr. Lamont expresses his conviction that the whole diurnal 

 movements are due to these earth-currents. He adds, however, that he 

 had hitherto been unable fully to verify this conclusion, owing to the con- 

 tinual changes produced in the collecting plates and in the wires by heat 

 and moisture. 



