1u May,1851, another United States ship, the John Adams, renewed the soundings of the Atlantic, and accomplished the greatest deep-sea sounding which had as yet been obtained. The bottom was found in latitude 32° 6' N., and longitude 44° 4 7' W., at 5500 fathoms (Lieutenant Walsh, in the Fancy, had reached 5700 fathoms, but found no bottom). In this great sounding two 32 lb. shot were used, which, together with the 5500 fathoms of line, were lost on attempting to recover them : correcting for drift, the actual depth is estimated by Captain Barron, her commander, at 4825 fathoms, or about 5i English miles.
It is unnecessary to follow up these observations in detail. The experience gained in them led to various contrivances whereby the saving of the line at the expense of the shot might be effected. In the Dolphin, U.S.N., an improvement was made in having shot so cast as to slide upon an irou rod. Upon the latter touching the bottom, the shot released itself, and the line and rod were more easily recovered.
A great advantage over the rod and shot was gained by the inven tion of a kind of double claw, acting somewhat upon the principle of that used for detaching the monkey of a pile-driving machine. It was invented by a blacksmith of Spitfire in 1851, named Carmelo Bonnici, a Maltese. Captain Spratt, R.N., who commanded the Spitfire, introduced it to public notice in the ' Nautical Magazine' for 1S56.
As the increasing demand for submarine telegraphs will much extend the work of deep-sea sounding, we give a few diagrams of the means employed in that interesting operation, as well as some sections of the sea bed, carefully drawn to a scale, and, in order to facilitate com parisons, each of them has the horizontal scale in proportion to the vertical as 30 to 1.
Bonnici's claw, as it is called, consists of two metal hooks of peculiar shape working on a pin, so that on any weight being attached it clasps and sustains a wire or line ; its great advantage being that any kind of sinker may be used with it, namely, either a shot or a pig of iron, or, on the want of these, a piece of heavy stone. In the above figures, No. I shows the outline of the apparatus when in course of descent, a a being the claws hooked into a rope-grummet, c, suspend ing a weight, b ; the arms, d d, of the claws lie close to the link e, to which is fastened the sounding-line f, and the whole outline is such as to favour the descent of the apparatus ; but the moment the lower portion touches the bottom, the arms d d being relieved of the weight of the sinker, drop, as in fig. 2, and the claws detach themselves from
the grummet, and the weight is left at the bottom.
It will be noticed that a piece projects beyond the underpart of the sinker which, among various contrivances, is loosely tied with string, g, to the body of the sinker : this has a hollow armed with tallow in the end, to bring up indications of the nature of the bottom bed, as in the ordinary hand-lead. On the sinker being detached from the apparatus, a small string, h, attached to the link of the instrument draws this clear of the mass of the sinker, and it is hauled up with the line.
This was a great step towards a better system, but it required con siderable care in setting the instrument, so as to insure the ready detachment of the weight on reaching the sea-bottom. Captain Spratt, however, made excellent use of it. He was engaged in sounding upon an extensive scale in the Mediterranean. In the accompanying section it will be seen that his greatest depth was 2170 fathoms, or nearly 2i miles : this was obtained in about lat. 35° 30' N., and long. IS° 45' E. It shows that eastward of Malta there exists a small plateau, having on it from 70 to 80 fathoms water, but that 50 or 60 miles from Malta, in the same direction, there is an enormous submarine escarpment of some 2000 fathoms, or 21 English miles.
In the following section we see that the bed of the Mediterranean rises as we proceed northward in the Archipelago, the deepest water (after passing the deeps about Caudia) being 420 fathoms, and then gradually shallowing.
In 1857, Captain T. Mansell, in H.M.S. Tartarus, was occupied in sounding between Alexandria, lihodes, and Smyrna. He also used Bonnici's claw, but had in addition Lieutenant Brooke's (U.S.N.) appa ratus in which had been substituted a conical mass of iron, sliding on a rod. This is an exceedingly simple and ingenious plan, but has been surpassed by improvements yet to be described. Captain Mansell seems to have given each of these instruments a fair trial in the Mediterranean, and records his decided preference for the Bonnici claw. The following will explain Brooke's apparatus. It is remark able as the one used in discovering the plateau existing between Ireland and Newfoundland.