Chase.] 264 [jan. 21, 



2. Magnetism. 



One of the fields in which tliere seems to be encouragement for much 

 special investigation, is Electro-magnetism. I gave many reasons, sixteen 

 years ago,* for regarding all magnetic phenomena as results of locally 

 modified vibrations, and on that account I have always thought it to be 

 somewhat illogical to speak of light as "an electro-magnetic phenomenon, ' ' 

 thus subordinating the general to the special. According to the law of 

 parsimony, tlie element of a common velocity indicates a common origin 

 in radiant energy, but in denoting that energy it seems better to use a name 

 which is universally recognized as appropriate, than one which may prob- 

 ably represent a mere local phenomenon. Among my reasons for adopting 

 this opinion was the delay in the manifestations of solar influence. Father 

 Secchi observed a remarkable solar outbreak on the 7th of July, 1872, 

 which produced a great agitation of terrestrial magnetism, and Airy esti- 

 mated the time required for the propagation of the magnetic disturbance 

 at 2h. 20m. This is 16.87 times the time required for light to come from 

 the Sun to the Earth, which is almost precisely the ratio between the time 

 of terrestrial rotation and the time of satellite revolution at the Earth's 

 surface. 



More than seven years after I had shown that the oolar daily variations 

 of terrestrial magnetism might be very closely represented, both in form 

 and magnitude, by solar tidal action. Maxwell suggested the same hypothe- 

 sis.f I showed, in 1875, :t^ that my hypothesis led to a simple estimate of 

 Sun's mass, which I placed upon record, without, however, giving all the 

 data for the calculation. The total terrestrial magnetic force in Great 

 Britain has been found to be lOgr. ft. sec. ; then the tension, perpendicular 

 to the lines of force, is .128 gr. weight per sq. ft. The greatest magnetic 

 tension which Joule was able to produce in Great Britain, by means of 

 electro-magnets, was about 140 lbs. weight on the sq. in. {■= 140 x 144 X 

 7000 = 141120000 gr. pr, sq. ft.). The unit of ms mm, in mechanical 

 measure, varying as the square of the unit of velocity, is 7n? P t*. But the 

 molecular oscillation, in alternate approach to and recess from the orbital 

 centre, continues for a half rotation or a half revolution, while the terres- 

 trial antagonism lasts only — as long. Therefore, if we designate Earth's 



mass by a subscript accent (mj), the ratio of the magnetic force which is 

 due to modified solar radiation, should bear the same ratio to the force of 

 terrestrial reaction, as w^ P t* : m^^ P t^ r*. or as m'^ : m^^ tt*. This gives the 

 proportion m'- : «i' tt* :: 141120000 :: 128, or m : m^ :: 327710 : 1. 



Although this result is nearly identical with the one deduced from the 

 comparative leverage of Sun and Jupiter (327897). in the paraboloid of in- 

 terstellar photodynamic action, I am inclined to think that it may be about 

 one per cent, too small. When the difficulty of making precise magnetic 



* lb. Vol. ix, etc. 



+ Treatise on Electricity and Magnetism, ii, 127. 



X Proc. Amer. Phil. Soc, xiv, 607-9. 



