316 Cambridge Philosophical Society : — 



After these preliminary explanations, a detailed account is given 

 of the mathematical investigation, and the paper, which was illus*; 

 trated with plans and geological sections, and a model of Arthur's 

 Seat, concludes with the following statement of the principal re* 

 suits r— * 



1 . " The effect of the attraction of the Pentland Hills is observed ? 

 in nearly equal amouA(Liat eiiohto£\tii»v^de #tatjOBS)Oni Arthui*s 

 Seat. '!-)/if,v yilDhJQOiy 'lo v:)i:riT«ifp ods (6 hi^iu-.ji'jm hi noil 



2. ''The calculated attractioDtf-fil^itfaftiiiiatt 0|£iAM^Tfflqfie^ 



the three stations are, — n\]omo-\\^'j[o Aoihi aib V) iii<ii b'^iUr-i'ji 



.. , >ili:>oo ^\— iUy.ivn o^oilv/' .liuDii- ' ' •; iil .noU 



South Statipi^,oi:r-vj{o ^^\?^t5d 1 "ti .stVorth StatiQn.bnj3 ,A= 



2"-25 Nort^-nuD 9r{9'^.?^3^«*^ dloHv/ V'-98 Soutb*r>ii3 oAi 



and, since the observed dlfl^ctipn at Aitttir's Seat is 5"* 27, the 

 apparent effect of the Pentlands is 4"' 93 at the summit of the hill. 



3. "Of this deflection of 4"*93, the computed attraction due to the 

 configuration of the ground within a radius of fifteen miles accounts 

 for about 2"* 6; and, inasmuch as we know that the igneous rocks 

 of Arthur's Seat and the Pentland Hills have an origin at a great 

 depth below the surface of the earth, the difference between the 

 observed and computed attraction is probably owing in part to the 

 high specific gravity of the mass of rock beneath them. ,; 



4. "The deflection at the Royal Observatory, Calton Hill, bein^ 

 5"- 63 South, exceeds that at Arthur's Seat by 0"-/0. Of this de;, 

 flection, 0"*60 is due to the configuration of the ground comprised- 

 within a circle of a mile and a quarter round the Observatory. 



5. "The latitude of Arthur's Seat or points in the neighbourhood 

 varies to the amount of 0"*02 between high and low water. 



6. "The mean density of the earth, determined from the observa-r 

 tions at the three stations on Arthur's Seat, is 5*14, with a probable, 

 error of +'07 due to the probable errors of the astronomical ampli- 

 tudes." "■ 



91ft -^iaaDJni hv loiiijupti •:>iiT ^ 



bnu ,ain9iijjoCAMBRiDGE philosophical sociETY^nol ai iBlimia 



^tContmued from vol. xi. p. 405.] ^^^^ ^^^^^^^,^^ ,,.j .^^^ 



Feb. 11, 1856. — A paper was read by Mr. Maxwell of Trinity 

 College, on Faraday's Lines of Force, in continuation of aiormet^^ 

 paper (Phil. Mag. vol. xi. p. 404). r i< j jxtjHoi-j rii '-'a .>loq oitsngiira 



This paper was chiefly occupied with "flie exteosioii' of tfcie tti€fthifd> 

 of lines of force to the phaenomena of electro- magnetism, by means^ 

 of a mathematical method founded on Faraday'eidea'of an "electro*.^' 

 tonic state." auin^im fci' Jiiioiio ' 



In order to obtain a clear view of the phaenomena to be isxplained,^ 

 we must begin with some general definitions of quantity and inten-- 

 sity as applicable to electric currents and to magnetic induction. It 

 was shown in the first part of this paper, that electrical and mag-- 

 netic phaenomena present a mathematical analogy to the case of a 

 fluid whose steady motion is afl^ected by certain moving forces and 

 resistances. [The purely imaginary nature of this fluid ha& been 



