124 Prof. Challis on the Zodiacal Light. 



light which I have so fully explained in the pages of this Journal. 

 The space through which the disturbances extend will only be 

 that which is occupied in common by the two steady motions, 

 which, without doubt, will be of much larger dimensions than the 

 space from which the light that is of sensible amount comes to 

 the eye of a spectator. Evidently, however, the form, position, 

 and density at different parts of this light will be just such as are 

 exhibited by the zodiacal light. Its principal plane will coincide 

 with the plane of the sun's equator ; which condition, as we have 

 seen, is at least very approximately fulfilled by the phenomenon ; 

 but, as before stated, the observations are not yet adequate to 

 decide whether or not it is exactly fulfilled. 



The facts and arguments that have now been adduced may 

 suffice for giving a distinct idea of the proposed theory of the 

 zodiacal light, and of the evidence on which it rests. It may 

 be mentioned as a circumstance confirmatory of the truth of 

 the theory, that it essentially depends on the reality of the 

 movement of the solar system, which movement accounts also 

 for the observed tendency of the proper motions of stars to mean 

 directions diverging from a certain point of the heavens. This 

 reference of widely different phenomena to a common cause is an 

 instance of that " consilience," which is one of Bacon's tests of 

 the truth of a theory. But not in this respect alone does the 

 proposed theory receive such confirmation. A considerable time 

 before I had concluded from observations that the form of the 

 zodiacal light is that which has been described in this commu- 

 nication, I had proposed a theory of magnetic force, in which 

 the cosmical magnetic influence resident in the sun, which has 

 been so satisfactorily educed by General Sabine from the discus- 

 sion of a large number of magnetic observations, was referred to 

 those very gyratory motions of the sether about the sun, of which, 

 according to the present theory, we have ocular evidence in the 

 zodiacal light. The magnetic theory is expounded in two com- 

 munications contained in the Numbers of the Philosophical 

 Magazine for January and February 1861, and the particular 

 applications alluded to are made in articles 34, 35, and 36. I 

 take this opportunity of adverting to the theoretical explanation 

 of General Sabine's "semiannual inequality" of the diurnal 

 variation of the declination, which is given very imperfectly in 

 art. 35, for the purpose of making an important addition to it, 

 which will at the same time serve to exhibit in a very distinct 

 manner the consilience of explanations just spoken of. As this 

 inequality is, at the same local hours, very nearly of the same 

 amount at places on the earth's surface widely different as to 

 latitude, it would seem to depend only on the earth's helio- 

 centric longitude. But since, according to the theory, no semi- 



