[11] Only very recently an asteroid, Hilda (153rd in order of detection), has been discovered which travels very much nearer to the path of Jupiter than to that of Mars—a solitary instance in that respect. Its distance (the earth’s distance being represented by unity), is 3·95, Jupiter’s being 5·20, and Mars’s 1·52; its period falls short of 8 years by only two months, the average period of the asteroidal family being only about 4½ years. Five others, Cybele, Freia, Sylvia, Camilla, and Hermione, travel rather nearer to Jupiter than to Mars; but the remaining 166 travel nearer to Mars, and most of them much nearer.
[12] Even this statement is not mathematically exact. If the rails are straight and parallel, the ratio of approach and recession of an engine on one line, towards or from an engine on the other, is never quite equal to the engines’ velocities added together; but the difference amounts practically to nothing, except when the engines are near each other.
[13] I have omitted all reference to details; but in reality the double battery was automatic, the motion of the observing telescope, as different colours of the spectrum were brought into view, setting all the prisms of the double battery into that precise position which causes them to show best each particular part of the spectrum thus brought into view. It is rather singular that the first view I ever had of the solar prominences, was obtained (at Dr. Huggins’s observatory) with this instrument of my own invention, which also was the first powerful spectroscope I had ever used or even seen.
[14] It varies more in some months than in others, as the moon’s orbit changes in shape under the various perturbing influences to which she is subject.
[15] It may seem strange to say that one hundred and twenty years after the passage of a comet which last passed in 1862, and was then first discovered, August meteors have been seen. But in reality, as we know the period of that comet to be about one hundred and thirty years, we know that the displays of the years 1840, 1841, etc., to 1850, must have followed the preceding passage by about that interval of time.
[16] The D line, properly speaking, as originally named by Fraunhofer, belongs to sodium. The line spoken of above as the sierra D line is one close by the sodium line, and mistaken for it when first seen in the spectrum of the coloured prominences as a bright line. It does not appear as a dark line in the solar spectrum.
[17] Since this was written, I have learned that Mr. Backhouse, of Sunderland, announced similar results to those obtained at Dunecht, as seen a fortnight or so earlier.
[18] Here no account is taken of the motions of the stars within the system; such motions must ordinarily be minute compared with the common motion of the system.
[19] Eight pictures of nebulæ were exhibited in illustration of this peculiarity.
[20] Sir John Herschel long since pointed to the variation of our sun as a possible cause of such changes of terrestrial climate.