, in Navigation, the act of trying the depth of the water, and the quality of the bottom, by a line and plummet, or other artifice.

At sea, there are two plummets used for this purpose, both shaped like the frustum of a cone or pyramid. One of these is called the hand-lead, weighing about 8 or 9lb; and the other the deep-sea-lead, weighing from 25 to 30lb. The former is used in shallow waters, and the latter at a great distance from the shore. The line of the hand-lead, is about 25 fathoms in length, and marked at every 2 or 3 fathoms, in this manner, viz, at 2 and 3 fathoms from the lead there are marks of black leather; at 5 fathoms a white rag, at 7 a red rag, at 10 and at 13 black leather, at 15 a white rag, and at 17 a red one.

Sounding with the hand-lead, which the seamen call heaving the lead, is generally performed by a man who stands in the main-chains to windward. Having the line all ready to run out, without interruption, he holds it nearly at the distance of a fathom from the plummet, and having swung the latter backwards and forwards three or four times, in order to acquire the greater velocity, he swings it round his head, and thence as far forward as is necessary; so that, by the lead's sinking whilst the ship advances, the line may be almost perpendicular when it reaches the bottom. The person sounding then proclaims the depth of the water in a kind of song resembling the cries of hawkers in a city; thus, if the mark of 5 be close to the surface of the water, he calls, ‘by the mark 5,’ and as there is no mark at 4, 6, 8, &c, he estimates those numbers, and calls, ‘by the dip four, &c.’ If he judges it to be a quarter or a half more than any particular number, he calls, ‘and a quarter 5,’ ‘and a half 4’ &c. If he conceives the depth to be three quarters more than a particular number, he calls it a quarter less than the next: thus, at 4 fathom 3/4, he calls, ‘a quarter less 5,’ and so on.

The deep-sea-lead line is marked with 2 knots at 20 fathom, 3 at 30, 4 at 40, &c to the end. It is also marked with a single knot at the middle of each interval, as at 25, 35, 45 fathoms, &c. To use this lead more effectually at sea, or in deep water on the sea-coast, it is usual previously to bring-to the ship, in order to retard her course: the lead is then thrown as far as possible from the ship on the line of her drift, so that, as it sinks, the ship drives more perpendicularly over it. The pilot feeling the lead strike the bottom, readily discovers the depth of the water by the mark on the line nearest its surface. The bottom of the lead, which is a little hollowed there for the purpose, being also well rubbed over with tallow, retains the distinguishing marks of the bottom, as shells, ooze, gravel, &c, which naturally adhere to it.

The depth of the water, and the nature of the ground, which are called the Soundings, are carefully marked in the log-book, as well to determine the distance of | the place from the shore, as to correct the observations of former pilots. Falconer.

For a machine to measure unfathomable depths of the sea, see Altitude.

Sounding the pump, at sea, is done by letting fall a small line, with some weight at the end, down into the pump, to know what depth of water there is in it.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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