Supposing the weight to have reached the bottom, and remained stationary, was it not probable that the line had been deflected from its vertical position by under-currents, with the result of greatly exaggerating the depth The immense lengths of line formerly reported to be run out con firmed what had been already guessed, viz., that in the deeper parts of the ocean the line does not cease to run out. And, as we 'have proved in the preceding article, the weight must have touched the bottom, probably at less than half the depth indicated by the length of line out. It must therefore have been deflected by under-currents.
The point to be determined, then, was—when did the weight touch the bottom Z To solve this apparently difficult point repeated experiments were made, and eventually the "law of descent" was discovered. It was found that the line did not run out at a uniform rate, but sank more slowly as the depth increased, and that the moment the weight touched the bottom, under-currents commenced to deflect the line at a uniform rate. So that, by carefully noticing the time when the line ceased to run out at a decreasing rate, and commenced running out regularly at a uniform rate, the time when the weight touched the bottom was easily ascertained, and the length of line run out thus far was evidently the actual depth at that point.
As regards the shifting of the station of observation during the "sounding," this was in a great measure obviated by sounding from a boat in still water ; boats being found to be less liable to "shift" than large vessels. Of course this diffi culty was experienced only in the open ocean, in the absence of any fixed objects whose bearings could be taken.
Concurrently with the desire for accurate information as to the depth of the sea, grew up the wish to know something of the nature of the floor of the ocean. The "sinkers" used in deep sea sounding were weights of lead or iron, necessarily heavy, in order to drag down the line to the bottom, but, once there, they could not be lifted up again, so that it appeared hopeless to think of obtaining any specimens from the bottom. One of Capt. Maury's coadjutors, Lieutenant Brooke, of the United States Navy, contrived a simple but most effective apparatus, by which the weight on touching the bottom was detached, leaving a light tube only to be drawn up. At the lower end of this tube a little tallow, or the barrel of a common quill, was attached, and to this specimens of the bottom adhered, and were thus brought up for examination. The instruments ased more recently on board the Challenger and other vessels were more finished and complicated, but the principle is the same—the disengagement of the weight from the line on touching the bottom. This also obviates the necessity of elaborate timing in order to determine the rate of descent, as the disengagement of the weight the moment it strikes the bottom relaxes the extreme tension of the line, which is then perfectly free.
But probably the most ingenious and perfect sounding apparatus yet invented, is that of Sir W. Thomson, which is founded on the fact that the pressure of the water increases with the depth in a definite ratio, so that, given the exact pressure, the actual depth is only a matter of calculation. This apparatus is thus described by Mr. Sewill, of Liverpool : " Sir William Thomson's Improved Sounding Machine is designed for the purpose of ascertaining, accurately and quickly, the depth of water under a ship, without stopping or even reducing her speed. The apparatus consists of a light wheel, on which is coiled about 300 fathoms of pianoforte steel wire. The stand to which the supports of the wheel are attached is fixed to the taffrail, at the stern of the vessel, so that the sinker can be left hanging ready for a cast. A cord wound round a groove in the circumference of the drum, is so arranged as to form a self-acting brake, which, when the sinker is hanging, offers enough of resistance to prevent it from run ning down, but when it is being hauled in, offers very little resistance to the turning of the wheel. When the order is given to take a sounding, the brake is released by the hand, so as to leave a force of about 7 lbs. pulling on the rope, by which a resistance of 5 lbs. is opposed to the wire while it is running out. Thus, when the sinker reaches the bottom, the wheel suddenly stops. The brake is then applied to prevent it from running on again. The sinker is a long iron weight of 22 lbs., with a hollow at the bottom to receive the usual arming of tallow, etc., to bring up a specimen of the bottom. It is attached to the end of the wire by means of a rope 6 or 8 feet long. A brass tube a little more than two feet long is lashed to the rope connecting the sinker with the wire. A glass tube, two feet long, coated inside with chromate of silver, and closed at one end and open at the other, is placed with its open end down, in the brass tube. As the sinker descends, the increased pressure drives the sea water up the glass tube, which, combining with the chromate of silver, makes a white mark. The height of the white mark registers the height to which the liquor has been forced up the tube. A scale, gra dusted to fathoms, shows at once the depth to which the glass tube has been. This method of taking soundings depends on the increased pressure as the tube descends, but is independent of the amount of line out, so that the vessel does not require to be stopped to take a sounding. The advantages of wire over rope of the same strength, are the smallness of area and smoothness of surface, on account of which it experiences very little resistance when pulled rapidly through water. This allows the sinker to descend very quickly, and to be hauled on board again with great ease, so that two men working the machine can take soundings in 100 fathoms every few minutes, from a ship running at any speed up to 16 or 17 knots."