132 



ON THE NUMERICAL RELATIONS OF GRAVITY AND MAGNETISM. 



probable connection of gravity and magnetism. At extra-tropical posts, the rotation-tide 

 becomes so preponderating that it is difficult to trace the diminished gravitation- and dif- 

 ferential-tides, or to discover any obvious numerical relations between the two great forces. 

 Still, the direction of the gravitating force furnishes so satisfactory an explanation of some 

 of the incidental phenomena of magnetic variation, that I am induced to extend my paper 

 somewhat, in order to direct attention to a field of research in which I believe physicists 

 may reasonably hope to obtain encouraging quantitative results. 



Among the pointings to unity of force which have led me to this belief, and have 

 strengthened my conviction that terrestrial magnetism is a simple reaction against dis- 

 turbances of terrestrial gravitation, are the following : Ampere's discovery of the magnetic 

 effect of electric currents circulating around iron bars ; Arago's experiments (which were 

 repeated and extended by Babbage, Herschel, Barlow, Christie, and others), showing that 

 simple rotation produces magnetic disturbances which are governed by fixed laws ; the 

 distribution of induced magnetism in masses of iron, which, as I announced at the So- 

 ciety's meeting of April 15, 1864, is the same as would follow from the relative centrifugal 

 motions of different portions of the earth, provided the magnetic axis corresponded with 

 the axis of rotation ; Hansteen's suspicion, confirmed by Sabine's practical demonstration* 

 of the influence of the sun upon terrestrial magnetism ; Sccchi's discovery that " the 

 diurnal excursion of the needle is the sum of two distinct excursions, of which the first 

 depends solely on the horary angle, and the second depends, besides, on the sun's declina- 

 tion,"! and that " all the phenomena hitherto known of the diurnal magnetic variations 

 may be explained by supposing that the sun acts upon the earth as a very powerful magnet 

 at a great distance •"% Captain Ross's observations upon the effect of barometric fluctua- 

 tions on the ocean-level (Proc. Hoy. Soc, June 15, 1854) ;§ Mr. Ferrel's paper on the 

 disturbance of barometric pressure by centrifugal force and friction (Nashville Journal of 

 Medicine and Surgery, 1856, and Mathematical Monthly for 1859, vol. 1, p. 140, sqq.) ;|| 

 and the various other considerations which I have hitherto adduced in support of my 

 views regarding the connection of rotation with aerial and retherial currents, and with 

 baric or magnetic perturbations. • 



The hypothesis that the sun has a specific magnetism, which acts upon the earth by 

 simple induction, like the earlier ones, which attributed terrestrial magnetism to one or 



* In his discussions of the observations at Toronto, Ilobavton, and St. Helena. 



t Phil. Mag. [4] 8, 396. % Ibid. 9, 452. § Ibid. 8, 318. 



|| I am glad to learn that Mr. Fcrrel has resumed his investigations with special reference to a fuller develop 

 merit of the theory of tidal action, and I have a confident hope that his researches will give us a clearer under- 

 standing of the phenomena of magnetic, as well as of oceanic tides. 



