The difficulty of sounding great depths arises from the fact that, after a large quantity of line has been run out, the shock of the lead striking the bottom cannot be felt. Moreover, there is sufficient force in the deep-sea currents to sweep out the line after the lead has reached the bottom so that, with the ordinary sounding-lines in use among navigators, it is impossible to sound great depths. Scientific men have, therefore, taxed their brains to invent instruments for sounding the deep sea--for touching the bottom in what sailors call "blue water." Some have tried it with a silk thread as a plumb-line, some with spun-yarn threads, and various other materials and contrivances. It has even been tried by exploding petards and ringing bells in the deep sea, when it was supposed that an echo or reverberation might be heard, and, from the known rate at which sound travels through water, the depth might thus be ascertained. Deep-sea leads have been constructed having a column of air in them, which, by compression, would show the aqueous pressure to which they had been subjected; but the trial proved to be more than the instrument could stand. Captain Maury, of the American Navy--whose interesting book has been already referred to--invented an instrument for sounding the deep sea. Here is his own description of it:--"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-registering 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 it up." One eccentric old sea captain proposed to sound the sea with a torpedo, or shell, which should explode the instant it touched the bottom. Another gentleman proposed to try it by the magnetic telegraph, and designed an instrument which should telegraph to the expectant measurers above how it was getting on in the depths below. But all these ingenious devices failed, and it is probable that the deepest parts of the ocean-bed still remained untouched by man.