Prof. Challis on the Zodiacal Light. 121 



sion that the coincidence of the principal plane of the zodiacal 

 light with the plane of the sun's equator may be assumed as a 

 probable deduction from observation, till the point be settled by- 

 additional observations of greater exactness than those hitherto 

 made. For greater accuracy, the observations should be taken, 

 if possible, by instrumental measurement, rather than by esti- 

 mation with reference to planets and bright stars. Good obser- 

 vations of the direction of the axis of the light, made in the 

 months of June and December, would most immediately tend to 

 decide this question. 



I have now to refer to certain observations of recent date which 

 have a bearing on the extent of the zodiacal light. Vol. iv. of 

 the 'Astronomical Journal/ edited by Dr. Gould, contains, in 

 p. 94, a letter dated May 17, 1855, addressed to the editor by 

 the Rev. George Jones, Chaplain on board the U.S. steam-frigate 

 c Mississippi/ in which he states that in a voyage chiefly in the 

 China and Japan seas, and extending from 41° north latitude to 

 50° south latitude, he had excellent opportunities for observation 

 of the zodiacal light. The observations are not given in detail, 

 but the following important statement is made : " I was also 

 fortunate enough to be twice near the latitude of 23° 28' north 

 when the sun was at the opposite solstice, in which position the 

 observer has the ecliptic at midnight at right angles with his 

 horizon, and bearing east and west. Whether the latter circum- 

 stance affected the result or not I cannot say ; but I then had 

 the extraordinary spectacle of the zodiacal light simultaneously 

 at both east and west horizons from 11 to 1 o'clock for several 

 nights in succession/'' From this observation, of the certainty 

 of which there seems to be no reason to doubt, it must be con- 

 cluded that the zodiacal light extends beyond the radius of the 

 earth's orbit ; for otherwise it would have been entirely below 

 the horizon of the observer at midnight. Consequently the earth 

 is either at all times enveloped by it, or, at least, when passing 

 across the line of its nodes. This fact, which stands directly 

 opposed to the idea which some have entertained that the zodiacal 

 light originates in a vast number of minute bodies circulating 

 about, and descending towards, the sun, necessitates the infer- 

 ence that it is simply luminosity. It will presently appear that 

 the theory I am now about to propose conducts to the same con- 

 clusion. 



The theory essentially rests on a law of the motion of fluids, 

 elastic or incompressible, which may be called the law of the 

 coexistence of steady motions. This law, I venture to predict 

 will eventually be found to have the same important bearing on 

 the theories of electricity, galvanism, and magnetism that the 

 law of the coexistence of small vibrations has in the theory of 



