The Depth of blue Water unknown, § 421. — Results of former Methods of Deep-sea Soundings not entitled to Confidence, 422. — Attempts by Sound and Pressure, 423. — The Myths of the Sea, 424. — Common Opinion as to its Depths, 425. — Interesting Subject, 427. — The deepest Soundings reported, 428. — Plan adopted in the American Navy, 429. — Soundings to be made from a Boat, 431. — Why the Sounding twine will not stop running out when the Plummet reaches Bottom, 432. — Indications of under Currents, 433. — Rate of Descent, 434. — Lieutenant John Mercer Brooke’s Deep-sea Sounding Apparatus, 437. — The greatest Depths at which Bottom has been found, 438.
421. Until the commencement of the plan of deep-sea soundings, as now conducted in the American Navy, the bottom of what the sailors call “blue water” was as unknown to us as is the interior of any of the planets of our system. Ross and Dupetit Thouars, with other officers of the English, French, and Dutch navies, had attempted to fathom the deep sea, some with silk threads, some with spun-yarn (coarse hemp threads twisted together), and some with the common lead and line of navigation. All of these attempts were made upon the supposition that when the lead reached the bottom, either a shock would be felt, or the line, becoming slack, would cease to run out.
422. The series of systematic experiments recently made upon this subject shows that there is no reliance to be placed on such a supposition, for the shock caused by striking bottom can not be communicated through very great depths, and therefore it does not follow that the line will become slack and cease to run out when the plummet reaches the bottom. Furthermore, the lights of experience show that, as a general rule, the under currents of the deep sea have force enough to take the line out long after the plummet has ceased to do so. Consequently, there is but little reliance to be placed upon deep-sea soundings of former methods, when the depths reported exceeded eight or ten thousand feet.
423. Attempts to fathom the ocean, both by sound and pressure, had been made, but in “blue water” every trial was only a failure repeated. The most ingenious and beautiful contrivances for deep-sea soundings were resorted to. By exploding heavy charges of powder in the deep sea, when the winds were hushed and all was still, the echo or reverberation from the bottom might, it was held, be heard, and the depth determined from the rate at which sound travels through water. But, though the explosion took place many feet below the surface, echo was silent, and no answer was received from the bottom.
Ericsson and others constructed deepsea leads having a column of air in them, which, by compression, would show the aqueous pressure to which they might be subjected. This was found to answer well for ordinary purposes, but in the depths of “blue water,” where the pressure would be equal to several hundred atmospheres, the trial was more than this instrument could stand. Mr. Baur, an ingenious mechanician of New York, constructed, according to a plan which I furnished him, a deep-sea sounding apparatus.
To the lead was attached, upon the principle of the screw propeller, a small piece of clock-work for registering the number of revolutions made by the little screw during the descent; and, it having been ascertained by experiment in shoal water that the apparatus, in descending, would cause the propeller to make one revolution for every fathom of perpendicular descent, hands provided with the power of self-registration were attached to a dial, and the instrument was complete. It worked beautifully in moderate depths, but failed in blue water, from the difficulty of hauling it up if the line used were small, and from the difficulty of getting it down if the line used were large enough to give the requisite strength for hauling up.
