In The Conflict Of Currents, As In The Oscillation Of The Waves, Our Imagination Is Struck By Those Movements Which Seem To Penetrate Each Other, And By Which The Ocean Is Continually Agitated.

We passed Cape St. Vincent, which is of basaltic formation, at the distance of more than eighty leagues.

It is not distinctly seen at a greater distance than 15 leagues, but the granitic mountain called the Foya de Monchique, situated near the Cape, is perceptible, as pilots allege, at the distance of 26 leagues. If this assertion be exact, the Foya is 700 toises (1363 metres), and consequently 116 toises (225 metres) higher than Vesuvius.

From Corunna to the 36th degree of latitude we had scarcely seen any organic being, excepting sea-swallows and a few dolphins. We looked in vain for sea-weeds (fuci) and mollusca, when on the 11th of June we were struck with a curious sight which afterwards was frequently renewed in the southern ocean. We entered on a zone where the whole sea was covered with a prodigious quantity of medusas. The vessel was almost becalmed, but the mollusca were borne towards the south-east, with a rapidity four times greater than the current. Their passage lasted near three quarters of an hour. We then perceived but a few scattered individuals, following the crowd at a distance as if tired with their journey. Do these animals come from the bottom of the sea, which is perhaps in these latitudes some thousand fathoms deep? or do they make distant voyages in shoals? We know that the mollusca haunt banks; and if the eight rocks, near the surface, which captain Vobonne mentions having seen in 1732, to the north of Porto Santo, really exist, we may suppose that this innumerable quantity of medusas had been thence detached; for we were but 28 leagues from the reef. We found, beside the Medusa aurita of Baster, and the Medusa pelagica of Bosc with eight tentacula (Pelagia denticulata, Peron), a third species which resembles the Medusa hysocella, and which Vandelli found at the mouth of the Tagus. It is known by its brownish-yellow colour, and by its tentacula, which are longer than the body. Several of these sea-nettles were four inches in diameter: their reflection was almost metallic: their changeable colours of violet and purple formed an agreeable contrast with the azure tint of the ocean.

In the midst of these medusas M. Bonpland observed bundles of Dagysa notata, a mollusc of a singular construction, which Sir Joseph Banks first discovered. These are small gelatinous bags, transparent, cylindrical, sometimes polygonal, thirteen lines long and two or three in diameter. These bags are open at both ends. In one of these openings, we observed a hyaline bladder, marked with a yellow spot. The cylinders lie longitudinally, one against another, like the cells of a bee-hive, and form chaplets from six to eight inches in length. I tried the galvanic electricity on these mollusca, but it produced no contraction. It appears that the genus dagysa, formed at the time of Cook's first voyage, belongs to the salpas (biphores of Bruguiere), to which M. Cuvier joins the Thalia of Brown, and the Tethys vagina of Tilesius. The salpas journey also by groups, joining in chaplets, as we have observed of the dagysa.

On the morning of the 13th of June, in 34 degrees 33 minutes latitude, we saw large masses of this last mollusc in its passage, the sea being perfectly calm. We observed during the night, that, of three species of medusas which we collected, none yielded any light but at the moment of a very slight shock. This property does not belong exclusively to the Medusa noctiluca, which Forskael has described in his Fauna Aegyptiaca, and which Gmelin has applied to the Medusa pelagica of Loefling, notwithstanding its red tentacula, and the brownish tuberosities of its body. If we place a very irritable medusa on a pewter plate, and strike against the plate with any sort of metal, the slight vibrations of the plate are sufficient to make this animal emit light. Sometimes, in galvanising the medusa, the phosphorescence appears at the moment that the chain closes, though the exciters are not in immediate contact with the organs of the animal. The fingers with which we touch it remain luminous for two or three minutes, as is observed in breaking the shell of the pholades. If we rub wood with the body of a medusa, and the part rubbed ceases shining, the phosphorescence returns if we pass a dry hand over the wood. When the light is extinguished a second time, it can no longer be reproduced, though the place rubbed be still humid and viscous. In what manner ought we to consider the effect of the friction, or that of the shock? This is a question of difficult solution. Is it a slight augmentation of temperature which favours the phosphorescence? or does the light return, because the surface is renewed, by putting the animal parts proper to disengage the phosphoric hydrogen in contact with the oxygen of the atmospheric air? I have proved by experiments published in 1797, that the shining of wood is extinguished in hydrogen gas, and in pure azotic gas, and that its light reappears whenever we mix with it the smallest bubble of oxygen gas. These facts, to which several others may be added, tend to explain the causes of the phosphorescence of the sea, and of that peculiar influence which the shock of the waves exercises on the production of light.

When we were between the island of Madeira and the coast of Africa, we had slight breezes and dead calms, very favourable for the magnetic observations, which occupied me during this passage. We were never weary of admiring the beauty of the nights; nothing can be compared to the transparency and serenity of an African sky. We were struck with the innumerable quantity of falling stars, which appeared at every instant.

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