na, 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.

The object in question was at no time, subsequently to 1805, visible to
the naked eye. Its aspect in Sir John Herschel's great reflector on the
23rd of September, 1832, was described by him as that of a "conspicuous
nebula," nearly 3 minutes in diameter. No trace of a tail was
discernible. While he was engaged in watching it, a small knot of minute
stars was directly traversed by it, "and when on the cluster," he tells
us,[254] it "presented the appearance of a nebula resolvable and partly
resolved into stars, the stars of the cluster being visible through the
comet." Yet the depth of cometary matter through which such faint
stellar rays penetrated undimmed, was, near the central parts of the
globe, not less than 50,000 miles.

It is curious to find that this seemingly harmless, and we may perhaps
add effete body, gave occasion to the first (and not the last) cometary
"scare" of an enlightened century. Its orbit, at the descending node,
may be said to have intersected that of the earth;