y
distinct substances. The long, straight ray which proceeded from the
comet of 1807, for example, was doubtless made up of particles subject
to a much more vigorous solar repulsion than those formed into the
shorter curved emanation issuing from it nearly in the same direction.
In the comet of 1811 he calculated that the particles expelled from the
head travelled to the remote extremity of the tail in eleven minutes,
indicating by this enormous rapidity of movement (comparable to that of
the transmission of light) the action of a force much more powerful than
the opposing one of gravity. The not uncommon phenomena of multiple
envelopes, on the other hand, he explained as due to the varying amounts
of repulsion exercised by the nucleus itself on the different kinds of
matter developed from it.

The movements and perturbations of the comet of 1811 were no less
profoundly studied by Argelander than its physical constitution by
Olbers. The orbit which he assigned to it is of such vast dimensions as
to require no less that 3,065 years for the completion of its circuit;
and to carry the body describing it at each revolution to fourteen times
the distance from the sun of the frigid Neptune. Thus, when it last
visited our neighbourhood, Achilles may have gazed on its imposing train
as he lay on the sands all night bewailing the loss of Patroclus; and
when it returns, it will perhaps be to shine upon the ruins of empires
and civilizations still deep buried among the secrets of the coming
time.[273]

On the 26th of June, 1819, while the head of a comet passed across the
face of the sun, the earth was in all probability involved in its tail.
But of this remarkable double event nothing was known until more than a
month later, when the fact of its past occurrence emerged from the
calculations of Olbers.[274] Nor had the comet itself been generally
visible previous to the first days of July. Several observers, however,
on the publication of these results, brought forward accounts of
singular spots perceived by them upon the sun at the time of the
transit, and an original drawing of one of them, by Pastorff of
Buchholtz, has been preserved. This undoubtedly authentic
delineation[275] represents a round nebulous object with a _bright_ spot
in the centre, of decidedly cometary aspect, and not in the least like
an ordinary solar "macula." Mr. Hind,[276] nevertheless, showed its
position on the sun to be irreconcilable with that which