We also took several of the shell-fishes, or testaceous animals, which are always found floating upon the water, particularly the Helix Janthina and Violacea; they 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 described according to Linnaeus’s system; he gave it the name of Larus crepidatus:  It is remarkable that the dung of this bird is of a lively red, somewhat like that of the liquor procured from the shells, only not so full; its principal food therefore is probably the Helix just mentioned.  A current to the N.W. prevailed more or less till Monday the 24th, when we were in latitude 1 deg. 7’ N. and longitude 28 deg. 50’.