"The old system is with a small line, marked at distances of one hundred fathoms, and with a weight of thirty or fifty pounds, the depth being told by the length of line run out. This is, of course, the most natural apparatus that suggests itself, and has been in use from the earliest ages. Experience has given directions for its use, avoiding some of the grosser causes of error from driftage and other causes. Yet its success in immense ocean depths is problematical, and a problem decided in the negative by many of the first scientific authorities at home and abroad. In the mechanical improvements of the last half-century substitutes for this simple but rather uncertain method began to be devised. It was proposed to ascertain the depth by the amount of pressure, or by explosions under water, with other equally impracticable plans. At last was noticed the perfect regularity of the movements of a spirally-shaped wheel, on being drawn through the water. Experiments proved that this regularity, when unaffected by other causes, could be relied on with perfect accuracy, and that an arrangement of cog-wheels would register its revolutions with mathematical precision. Very soon it came in use as a ship's log. So perfect was their precision, that they were even introduced in scientific surveys. Base lines, where the nicest accuracy is required, were run with them, and we have the highest authority of the Royal Navy for believing that they never failed. At this point it was proposed to apply them in a perpendicular as well as in a horizontal motion through the water. Massey's apparatus promising to solve those problems of submarine geography left unsolved by the old method of obtaining depth with a simple line and sinker, and this more especially as some causes of error, considerable on the surface, disappear in the still water below." To make our knowledge of the sea complete, one thing more was wanting--a method not only of reaching the bottom, but of laying hold of it, and bringing it up to the light of day. This was now to be supplied. [Illustration: BROOKE'S DEEP SEA SOUNDING APPARATUS. A shows the instrument ready for sounding. It is very simple, consisting only of a cannon-ball, pierced with an iron rod, and held in its place by slings. As the ball goes down swiftly, it drives the rod into the bottom like the point of a spear, when an opening at the end catches the ooze in its iron lips. The same instant (see B) the slings loosen, the ball drops off, and the naked rod, C, with its "bite" is drawn up to the surface.]