The character of the supposed resistance in inter-planetary space has, it may be remarked, been often misapprehended. What Encke stipulated for was not a medium equally diffused throughout the visible universe, such as the ethereal vehicle of the vibrations of light, but a rare fluid, rapidly increasing in density towards the sun.[251] This cannot be a solar atmosphere, since it is mathematically certain, as Laplace has shown,[252] that no envelope partaking of the sun's axial rotation can extend farther from his surface than nine-tenths of the mean distance of Mercury; while physical evidence assures us that the _actual_ depth of the solar atmosphere bears a very minute proportion to the _possible_ depth theoretically assigned to it. That matter, however, not atmospheric in its nature--that is, neither forming one body with the sun nor altogether aëriform--exists in its neighbourhood, can admit of no reasonable doubt. The great lens-shaped mass of the zodiacal light, stretching out at times far beyond the earth's orbit, may indeed be regarded as an extension of the corona, the streamers of which themselves mark the wide diffusion, all round the solar globe, of granular or gaseous materials. Yet comets have been known to penetrate the sphere occupied by them without perceptible loss of velocity. The hypothesis, then, of a resisting medium receives at present no countenance from the movements of comets, whether of short or of long periods. Although Encke's comet has made thirty-five complete rounds of its orbit since its first detection in 1786, it shows no certain signs of decay. Variations in its brightness are, it is true, conspicuous, but they do not proceed continuously.[253] The history of the next known planet-like comet has proved of even more curious interest than that of the first. It was discovered by an Austrian officer named Wilhelm von Biela at Josephstadt in Bohemia, February 27, 1826, and ten days later by the French astronomer Gambart at Marseilles. Both observers computed its orbit, showed its remarkable similarity to that traversed by comets visible in 1772 and 1805, and connected them together as previous appearances of the body just detected by assigning to its revolutions a period of between six and seven years. The two brief letters conveying these strikingly similar inferences were printed side by side in the same number of the _Astronomische Nachrichten_ (No. 94); but Biela's priority in the discovery of the comet was justly recognised by the bestowal upon it of his name.