469 



of 85°, instead of being nearly circular, as represented in the 3*^ 

 Abtheilung of Plate XVI. of the ' Atlas des Erdmagnetismus,' is an 

 elongated ellipse, much more nearly resembling in form and dimen- 

 sions the ellipse of 85° of inclination in the northern hemisphere in 

 the same M^ork, Plate XVI. Abtheilung. The analogy between 

 the two hemispheres in the characteristic feature of the elliptical 

 form of the higher isoclinal lines is the more important to notice, on 

 account of the particular relation which appears to subsist in the 

 northern hemisphere between the change in the geographical direc- 

 tion of the greater axis of the ellipse, and the secular changes of the 

 inclination generally throughout the hemisphere. The present di- 

 rection of the greater axis in the northern hemisphere, is nearly 

 N.N.W. and S.S.E., or that of a great circle passing through the 

 two foci of maximum intensity. In the southern hemisphere, the 

 present direction of the greater axis differs little from E.S.E. and 

 W.N.W. 



3. Captain Ross's observations of the intensity do not appear to 

 indicate the existence anywhere in the southern hemisphere of a 

 higher intensity than would be expressed by 2*1 of the arbitrary 

 scale. In this respect also the analogy between the two hemi- 

 spheres appears to be closer than is shown in M. Gauss's maps, 

 Plate XVIII. Vvith respect to the direction of as much of the line 

 of highest intensity (2*0) as it has been possible to draw with any 

 degree of confidence from the observations now communicated, it 

 will be found to be in almost exact parallelism with the isodynamic 

 line of 1*7 in Plate III. of the author's report " On the Variations of 

 the Magnetic Intensity," in the Report of the eighth meeting of the 

 British Association, for 1 838 ; which line was the highest of which 

 the position could be assigned at that period for any considerable 

 distance by the aid of the then existing determinations. 



3. " An Account of several new Instruments and Processes for 

 determining the Constants of a Voltaic Circuit," by Charles Wheat- 

 stone, V.P.R.S., Professor of Experimental Philosophy in King's 

 College, London, Corresponding Member of the Royal Academy of 

 Sciences at Paris, &c. 



The author proposes in the present communication to give an ac- 

 count of various instruments and processes which he has employed 

 during several years past for the purpose of investigating the laws 

 of electric currents. He states that the practical object for which 

 these instruments were originally constructed, was to ascertain the 

 most advantageous conditions for the production of electric effects 

 through circuits of great extent, in order to determine the practica- 

 bility of communicating signals by means of electric currents to more 

 considerable distances than had hitherto been attempted. Their use, 

 however, is not limited to this special object, but extends equally to 

 all inquiries relating to the laws of electric currents and to every 

 practical application of this wonderful agent. 



As the instruments and processes described by the author are all 

 founded on Ohm's theory of the voltaic circuit, he commences with 



