The investigation is even now by no means complete, though it has been going on for many years. It has, however, been ascertained that, while the ocean-bed deepens more or less rapidly as we recede from the shore, forming a trough in which the Gulf Stream flows, this trough is limited on its eastern side by a range of hills trending in the direction of the current, outside of which is another depression or valley. Indeed, the sea-bottom exhibits parallel ridges and depressions, running like the shore of the continent itself, in a northeasterly direction. The water presents differences of temperature, not only on the surface, but at various depths below. These inequalities have been determined by a succession of thermometric observations along several lines, crossing the Gulf Stream from the shore to the ocean water on its eastern side, at intervals of about a hundred miles. The observations have been made first at the surface, and then at successively greater depths, varying from ten to twenty, thirty, one hundred, two hundred, and even three and four hundred fathoms. This survey has shown that, while the Gulf Stream has a temperature higher than that of the waters on either side, it is also alternately warmer and colder within itself, being made up as it were of distinct streaks of water of different temperature. These alternations continue to as great a depth as the observations have been carried, and are found to extend even to the very bottom of the sea, where this has been reached. The most surprising part of this result is the abruptness of the change along the line where the two great currents touch each other. So sharp is this division that the boundary of the Arctic current is now technically designated as the “Cold wall” of the Gulf Stream. Of course as the latter flows northward and eastward it gradually widens, and its temperature is lowered; but even as far north as Sandy Hook the difference between its temperature at the surface and that of the surrounding waters is still marked.

Off Cape Florida the width of the Gulf Stream is not over forty miles; off Charleston it is one hundred and fifty miles; while at Sandy Hook it exceeds three hundred miles.

The inequality of the bottom may be appreciated by the soundings off Charleston, where, from the shore to a distance of two hundred miles, the following depth was successively measured: 10, 25, 100, 250, 300, 600, 350, 550, 450, 475, 450, and 400 fathoms.

The following table may give some idea of the temperature of the stream in connection with its depth:—

Off Sandy Hook, at successive distances from the coast, of
100,150,200,250,300,350,and400miles,
the temperature near the surface to a depth of thirty fathoms averages:
65°,66°,64°,81°,80°,and75°Fahr.;
at a depth of between forty and a hundred fathoms it averages:
50°,52°,50°,47°,72°,68°,and65°Fahr.;
at a depth below three hundred fathoms it averages:
37°,39°,40°,37°,55°,57°,and55°Fahr.

The rapid rise of the temperature after the fourth column of figures indicates the position of the Cold wall.

For further details see the United States Coast Survey Report for 1860, page 165, and the accompanying maps,—which should be copied into all our school atlases.

II.—FLYING-FISHES.

The motions of animals vary greatly with reference to the medium in which they live. Our present knowledge renders it, however, necessary that we should weigh these differences with reference to the structural character of the organs of locomotion themselves, as well as to that of the peculiar resistance of the element in which they move. When we speak of the flight of Birds, of Insects, of Fishes, of Bats, &c., and designate their locomotive organs indiscriminately as wings, it is evident that the character of the motion and not the special structure of the organs has determined our nomenclature. We are influenced by the same consideration when we give the name of fins to the organs of all animals which swim in the water, be they Whales, Turtles, Fishes, Crustacea, or Mollusks. It requires but a superficial acquaintance with the anatomy of the flying-fishes to perceive that their organs of flight are built upon exactly the same pattern as the pectoral fins of most fishes, and differ entirely from the wing of birds, as also from the wing of bats, the latter being in all essentials a paw, identical with the paw of ordinary quadrupeds, save the length of the fingers and the absence of nails on the longest of them. No wonder, then, that the flight of the flying-fishes should entirely differ from that of birds or bats.

I have had frequent occasions to observe the flying-fishes attentively. I am confident not only that they change the direction of their flight, but that they raise or lower their line of movement repeatedly, without returning to the water. I avoid the word falling designedly, for all the acts of these fishes during their flight seem to me completely voluntary. They raise themselves from the surface of the water by rapidly repeated blows with the tail, and more than once have I seen them descend again to the surface of the water in order to repeat this movement; thus renewing the impulse and enabling themselves to continue for a longer time their passage through the air. Their changes of direction, either to the right and left or in rising and descending, are not due to the beating of the wings, that is to say, of the great pectoral fins, but simply to an inflexion of the whole surface, in one or the other direction, by the contraction of the muscles controlling the action of the fin-rays, their pressure against the air determining the movement. The flying-fish is in fact a living shuttlecock, capable of directing its own course by the bending of its large fins. It probably maintains itself in the air until the necessity of breathing compels it to return to the water. The motive of its flight seems to me to be fear; for it is always in the immediate neighborhood and in front of the vessel that they are seen to rise; or perhaps at a distance when they are pursued by some large fish. Now that I have studied their movements, I am better able to appreciate the peculiarities of their structure, especially the inequality of the caudal fin. It is perfectly clear that the greater length of the lower lobe of the caudal is intended to facilitate the movements by which the whole body is thrown out of water and carried through the air; while the amplitude of the pectoral fins affords only a support during the passage through the lighter medium. Nothing shows more plainly the freedom of their movements than the fact that, when the surface of the sea is swelling into billows, the flying-fishes may hug its inequalities very closely and do not move in a regular curve, first ascending from and then descending again to the level of the water. Nor do they appear to fall into their natural element, as if the power that had impelled them was exhausted; they seem rather to dive voluntarily into the water, sometimes after a very short and sometimes after a rather protracted flight, during which they may change their direction, as well as the height at which they move.