Thus the mean of our experiments gave us, for the sinker and twine used,
| 2 m. 21 s. | as the | average time | of descent from | 400 to 500 | fathoms. |
| ,, | ,, | ,, | ,, | 1000 to 1100 | ,, |
| 4 m. 29 s. | ,, | ,, | ,, | 1800 to 1900 | ,, |
572. The law of the plummets descent.—Now, by aid of the law here indicated, we could tell very nearly when the ball ceased to carry the line out, and when, of course, it began to go out in obedience to the current and drift alone; for currents would sweep the line out at a uniform rate, while the cannonball would drag it out at a decreasing rate. The development of this law was certainly an achievement, for it enabled us to show that the depth of the sea at the places named (§ 567) was not as great as reports made it. These researches were interesting: the problem in hand was important, and it deserved every effort that ingenuity could suggest for reducing it to a satisfactory solution.
573. Brooke's sounding apparatus.—As yet no specimens of the bottom had been brought up. The line was too small, the shot was too heavy, and it could not be weighed; and if we could reach the bottom, why should w^e not know its character? In this state of the case. Passed Midshipman J. M. Brooke, United States Navy, who at the time was associated with me on duty at the Observatory, proposed a contrivance by which the shot, on striking the bottom, would detach itself from the line, and send up a specimen of the bottom. This beautiful contrivance, called Brooke's Deep-sea Sounding Apparatus, is represented on p. 313. A is a cannon-ball, having a hole through it for the rod B. Figure 1 represents the rod B, and the slings D D, with the shot slung, ready for sounding. Figure 2 represents the apparatus in the act of striking the bottom; it shows how the shot is detached, and how specimens of the bottom are brought up, by adhering to a little soap or tallow,[1] called "arming," in the cup C, at the lower end of the rod B. With this contrivance specimens of the bottom have been brought up from the depth of nearly four miles.
- ↑ The barrel of a common quill attached to the rod has been found to answer better.
