Page:Physical Geography of the Sea and its Meteorology.djvu/336

310 approbation of the Congress of the United States; for that body, in a spirit worthy of the representatives of a free and enlightened people, had authorized the Secretary of the Navy to employ three public vessels to assist in perfecting the discoveries, and in conducting the investigations connected therewith.

668. The plan finally adopted.—The plan of deep-sea soundings finally adopted, and now in practice, is this: Every vessel of the navy, when she puts to sea, is, if she desires it, furnished with a sufficient quantity of sounding-twine, carefully marked at every length of one hundred fathoms—six hundred feet—and wound on reels of ten thousand fathoms each. It is made the duty of the commander to avail himself of every favourable opportunity to try the depth of the ocean, whenever he may find himself out upon "blue water," For this purpose he is to use a cannon-ball of 32 or 68 pounds as a plummet. Having one end of the twine attached to it, the cannon-ball is to be thrown overboard from a boat or a steamer, and suffered to take the twine from the reel as fast as it will. The reel is made to turn easily A silk thread, or the common wrapping-twine of the shops, would, it was thought, be strong enough for this purpose, for it was supposed there would be no strain upon the line except the very slight one required to drag it down, and the twine having nearly the specific gravity of sea water, this strain would, it was imagined, be very slight. Moreover, when the shot reached the bottom, the line, it was thought (§ 561), would cease to run out; then breaking it off, and seeing how much remained upon the reel, the depth of the sea could be ascertained at any place and time simply at the expense of one cannon-ball and a few pounds of common twine.

669. Discovery of currents in the depths of the sea.—But practical difficulties that were not expected at all were lurking in the way, and afterwards showed themselves at every attempt to sound; and it was before these practical difficulties had been fairly overcome that the great soundings (§ 667) were reported. In the first place, it was discovered that the line, once started and dragged down into the depths of the ocean, never would cease to run out (§ 662), and, consequently, that there was no means of knowing when, if ever, the shot had reached the bottom. And, in the next place, it was ascertained that the ordinary twine (§ 666) would not do; that the sounding-line, in going down, was really subjected to quite a heavy strain,