y are about the size of a snail, and are
supported upon the surface of the water by a small cluster of bubbles,
which are filled with air, and consist of a tenacious slimy substance
that will not easily part with its contents; the animal is oviparous,
and these bubbles serve also as a _nidus_ for its eggs. It is probable
that it never goes down to the bottom, nor willingly approaches any
shore; for the shell is exceedingly brittle, and that of few fresh-water
snails is so thin: Every shell contains about a tea-spoonful of liquor,
which it easily discharges upon being touched, and which is of the most
beautiful red-purple that can be conceived. It dies linen cloth, and it
may perhaps be worth enquiry, as the shell is certainly found in the
Mediterranean, whether it be not the _Purpura_ of the ancients.[69]

[Footnote 69: It is quite impossible to discuss this subject here. But
it may be worth while to refer the learned reader for some curious
information about it, to the illustrious Bochart's work entitled
Hierozoicon, Part II. Book V. Ch. II. There are several sorts of
sea-shells, that yield the purple-dye so much esteemed among the
ancients. Pliny, who has written on the subject, divides them into two
classes, the _buccinum and purpura_, of which the latter was most in
request. According to him, the best kinds were found in the vicinity of
Tyre. That city was famous for the manufacture of purple. To be _Tyrio
conspectus in ostro_, seemed, in the estimation of the Mantuan poet,
essential to his due appearance in honour of Augustus, Geor. 3--17. But
several other places in the Mediterranean afforded this precious
article. Thus Horace speaks of Spartan purple,

Nec _Laconicas_ mihi
Trahunt honestae _purpuras_ clientae.

Od. Lib. 2. 18.

The English reader will be much pleased with several interesting remarks
as to the purple and other colours known to the ancients, given in
President Goguet's valuable work on the origin of laws, arts. &c. &c. of
which a translation by Dr Henry was published at Edinburgh 1761.--E.]

On the 8th, in latitude 8 deg. 25' north, longitude 22 deg. 4' west, we found a
current setting to the southward, which, the next day in latitude 7 deg.
58', longitude 22 deg. 13', shifted to the N.N.W. 1/4 W. at the rate of one
mile and a furlong an hour. The variation here, by the mean of several
azimuths, appeared to be 8 deg. 39' W.

On the 10th, Mr Banks shot the black-toed gull, not yet descri