34 



metallic body in the neighbourhood of a magnet was made 

 perfectly manifest. If then there is any magnetic relation 

 whatever between the sun and comet, the latter will cer- 

 tainly experience resistance. 



The question is thus resolved into one concerning the pro- 

 bability that a comet would experience electric disturbance 

 in approaching the sun. On this point we have the evidence 

 now existing that there is a close magnetic relation between 

 the sun and planets. If, as is generally believed, the sun- 

 spot periods depend on the motions of the planets, a 

 small fraction of the planetary energy must be expended. 

 I find, indeed, that a very brief remark to this effect was 

 given in the memoir of the original discoverers of the rela- 

 tion, namely, Messrs. Warren de la Rue, Balfour Stewart, 

 and B. Loewy. At p. 45 of their Eesearches on Solar 

 Physics they add a small note to the following effect : " It 

 is, however, a possible enquiry whether these phenomena 

 do not imply a certain loss of motion in the influencing 

 planets." As I conceive, no doubt can exist that periodic 

 disturbances depending upon the motions of bodies must 

 cause a certain dissipation of their energy, for if stationary 

 the constant radiation of the sun could not produce any 

 periodic changes, unless the sun were itself variable. Is 

 there not then a reasonable probability that the light of 

 the Aurora represents an almost infinitesimal fraction of 

 the earth's energy, and that in like manner the light of 

 Encke's comet represents a far larger fraction of its energy ? 

 It is also worthy of notice that the tail of a comet is usually 

 developed most largely at those parts of its orbit where the 

 rate of approach or recess is most rapid, and where the 

 electric disturbance would be correspondingly intense. 



I do not, of course, deny that the resisting medium may 

 nevertheless exist, or may by other observations or experi- 

 ments be made manifest. But I hold that so long as other 

 physical causes can be pointed out which might produce 



