Exceedingly beautiful in the tropic seas are those small physali, a species of Acalephæ, known to sailors as "Portuguese men-of-war." They sail along by means of their large air-bags, exposed to the wind, whilst their numerous long, dark-blue tentacles, like fibres or roots, reach deep into the water, extending or contracting in order to secure their food. The fringes of these air-bags are of a violet colour, and resemble, when in sunshine, a splendid flower, floating about the sea in all directions.
Approaching the zone of the trade-winds, the aspect of the sea is animated by flying-fish. It has been long a matter of doubt whether this fish really moves through the air by flying, or whether, being pursued by enemies, it merely darts from fear out of the water, and sustains itself in the air by means of its long pectoral fins, though the long distances it moves are out of all proportion to its size and probable muscular power. This doubt seems to have originated with those who never have had an opportunity of personally observing flying-fish; or who have merely seen these creatures in the Mediterranean, where they are so small as to render it rather difficult to decide in what the movement really consists.
But in the tropics all doubt vanishes, for the fish here attains a length of from 6 to 8 inches, and its pectoral fins are so much developed that they may perfectly well be used as wings. By closely observing them, it is seen that these fins, or wings, have an oscillating or vibratory motion, like that of the grasshopper, by which they assist the oblique spring from the water, produced by muscular power; they move at a height of from two to three feet above the surface of the sea, to a distance of from 50 to 100 yards, and at the same time change the direction of their flight in exactly the same way as grasshoppers.
It seems, however, to be neither pleasure nor want of food that drives these creatures from their own element. They have numerous enemies, of which the principal is the bonita (Scomber Palamy); whenever flying-fish were seen the bonitas likewise made their appearance; we often saw the latter darting out of the water and snapping at a flying-fish, when, if they did not succeed, they were perceived hastening under water in the same direction that their prey was taking. But the flying-fish is quite aware of the movement of the enemy, and, therefore, sometimes turns back abruptly, evidently with a view of dodging his pursuer. The advantage of the wings is, however, more apparent than real, for whilst they escape by these means from the bonitas, dorados, and dolphins, they fall a sacrifice to the seagulls and man-of-war birds (Tachypetes aquilus), which reach them like arrows shot from a bow, before they are able to conceal themselves again in the ocean. Those which were seen on the north of the equator, were invariably of the smaller kind (Exocætus volitans); whilst in the south, also, the larger species (Exocætus exulans) made their appearance. They often drop during the night on the decks of small vessels with low bows, but on the Novara, which rose about 20 feet over the surface of the water, this was not possible; however, some fell upon the chains, which were caught and preserved in alcohol by the zoologists.
On the 27th June, at midday, we had arrived in 27° 2′ N. Lat., and 24° 7′ W. Long., and lay in a perfect calm, the mirror-like aspect of the sea allowing us to take a deep-sea sounding with what is called Brooke's apparatus. This consists of a tube, which runs through a thirty-pound shot, fastened to a thin line. As the shot, however, cannot easily be drawn up after the sounding, there is an ingenious mechanism, by means of which, on touching the ground, it is detached from the tube, which may then be drawn up, the shot remaining in the sea. The tube has a funnel-shaped opening at its lower extremity, by which a portion of the ground may likewise be brought up for scientific examination.
In the afternoon a boat provided with everything requisite for the sounding, was launched, and the operation began. Unfortunately there were only 4050 fathoms of line, as we did not find at Gibraltar the quality required for the purpose. The whole stock ran entirely out without reaching ground; we could therefore do nothing more than make the attempt to heave the tube without the shot. In this, however, we succeeded only in so far as regarded the first 2000 fathoms; then the line broke, and the remainder was lost. One of the causes of this accident was the sudden rise of the trade-wind which increased so much as to engross nearly our entire attention.
To convey an idea of the time a ball of this kind takes to descend through the different strata of water, which increases in density in proportion to the depth attained, we annex the length of the different periods required in paying out the line during the experiment:—
| Minutes. | Seconds. | |||
| The first 1000 | fathoms occupied | 19 | 8 | |
| From | 1000 to 2000 | " | 21 | 3 |
| " | 2000 to 3000 | " | 40 | 6 |
| " | 3000 to 4000 | " | 76 | 6 |
| " | 4000 to 4050 | " | 3 | 5 |
| _______ | _______ | |||
| Total time in running out the entire length | 2h 39m | 28s |
As the winding up of the tube requires at least the same period, it may easily be conceived how great is the expenditure of time necessary for such an experiment, and on what chances the success of the whole operation depends. Though this trial was partially unsuccessful, yet so much is certain, that at this point, 24,300 feet of line were run off without reaching the ground. There is always, however, to be taken into consideration the under-current, which draws the line out of the perpendicular, and renders the result of the operation rather doubtful.
We now sailed with a fresh and steady trade-wind towards the thirtieth degree of W. Long., and thence along that meridian to the southward.