HISTORY OF PHYSICAL ASTRONOMY.

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merits far superior in power to any that had been hitherto used for asimilar purpose, and with the scrupulous attention to accuracy which cha-racterized all the observations of that illustrious astronomer. He sup-posed the bright appearances around the poles to be the light reflectedfrom masses of ice and snow accumulated iu the polar regions; and heascribed their occasional diminution to the dissolving influence of thesolar rays, when they became exposed to them by the revolution of theplanet in its orbit. The changes actually exhibited by the spots affordeda striking confirmation of this view of their origin. Thus, in the year1781, the spot about the south pole was extremely large, but this mightnaturally have been expected, since the pole had been recently involved inperpetual darkness during the space of twelve months. In 1783 it hadbecome considerably smaller, and it continued to decrease from the 20iliMay of that year till about the middle of September. During the last-mentioned interval, the south pole had already been above eight monthsenjoying the benefit of summer, and still continued to receive the sun-beams, though, towards the close of the period, in so oblique a direction asto derive but little benefit from them. On the other hand, in the year1781, the north polar spot which had been enjoying the sunshine fortwelve months, and was but lately returning to darkness, appeared small,though increasing in size. It was indeed invisible in the year 1783, butthis arose from the position of the axis of the planet, by which it v'asremoved out of sight*. The explanation of Sir William Herschel has beengenerally adopted by astronomers, as the most probable that can be offeredrespecting the interesting phenomena to which allusion has just been made.It is hardly necessary to remark that such an explanation implies the ex-istence of an atmosphere about the planet.

If we consider the arrangement of the planetary system as it was knownprevious to the discovery of Uranus , it will readily occur, either from a com-parison of the numerical values of the mean distances of the planets, orfrom an inspection of their orbits when traced concentrically by a graphicprocess on a sheet of paper, that while the intervals between the succes-sive orbits increase continually outwards from the sun, a vacuity, dispro-portionately larger than any of the others, exists between the orbits of Mars and Jupiter . When Kepler, during the earlier period of his astronomicalcareer, was endeavouring to discover a connexion between the meandistances of the planets from the sun, and their motions in their orbits,he encountered in this anomalous fact an insuperable obstacle to thesuccess of bis speculations. The most plausible hypothesis that his fertileimagination could suggest, appeared to require that the vacuity should befilled up by the interposition of another orbit. At length, having despairedof reconciling the actual state of the planetary system with any theory hecould form respecting it, he hazarded the assertion that a planet reallyexisted between the orbits of Mars and Jupiter , and that its smallnessalone prevented it from being visible to astronomers. This bold surmiseof the illustrious discoverer of the laws of the planetary movements, wasgenerally regarded as not the least probable of those transient sallies of theimagination in which he delighted to indulge, and it was favourably alludedto by Lambert in his “ Cosmological Letters,” published in 1769. In 1772,Bode published a treatise on astronomy in which he first announced thesingular relation between the mean distances of the planets from the sun,which has since been distinguished by his name. This relation, to which* Phil. Trans., 1784, p. 261.