OF ARTS AND SCIENCES. 149 



The first deduction which Jones himself made from his observations 

 (Jj., Introduction, xvi.) was that the place of the zodiacal light with 

 respect to the ecliptic depended upon the position of the ecliptic in 

 the visible hemisphere ; so that the light was comparatively deficient 

 on the lower side of the ecliptic. As Jones states it, when he was 

 north of the ecliptic, the main body of the light was also north of it ; 

 when he was south of the ecliptic, so was the main body of the light ; 

 when he was nearly on the ecliptic, the light was equally divided by 

 the ecliptic, or nearly so. He also found that in a large majority of 

 cases a change in the zenith distance of the pole of the ecliptic during 

 a single evening was accompanied by a progressive change of the 

 zodiacal light, in accordance with the rule just stated. 



A cause, obviously tending to produce an effect of this kind, is well 

 known to exist. The atmosphere of the Earth absorbs light with in- 

 creasing efficiency as the line of sight descends from the zenith 

 towards the horizon. Accordingly, the brightness of the zodiacal 

 light on the lower side of the ecliptic should be diminished relatively 

 to that of the light at the same distance from the ecliptic on the upper 

 side. But before accepting this explanation of the law discovered by 

 Jones, we must see whether the law itself is confirmed by the expe- 

 rience of other observers, and whether the known amounts of atmos- 

 pheric absorption at diflferent altitudes exhibit a reasonable degree 

 of correspondence with the observed effects. The subject has been 

 debated by Geelmuyden and Groneman (G. 83), but seems to need 

 the different treatment here proposed. 



A general view of the result for the elongation 60° is afforded by 

 Table I., below. The first two columns, headed "Observer" and 

 " Group," relate to the arrangement of the observations m the final 

 tables of the Appendix. The " Diffuse " Light observed by Jones is 

 not here taken into consideration. The next four columns contain the 

 latitude of the observer's station ; the zenith distance of the north pole 

 of the ecliptic, here called the inclination of the ecliptic; the elonga- 

 tion of the zenith, by which is meant the excess of its longitude over 

 that of the Sun ; and. finally, the zenith distance of the Sun. The next 

 column gives the number of observations upon which the preceding 

 results directly or indirectly depend, as is explained in the Appendix. 

 The remaining columns relate to the position of the zodiacal light with 

 respect to the ecliptic, 60° from the Sun. The first of these columns 

 contains the number of observations upon which the result depends. 

 A strict reduction would have required the number of the preceding 

 column always to agree with this ; that is, the data for the position of 



