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Soundings, to ascertain the depth of water on entering or leaving port, or in any case where there is supposed to be less than 15 fathoms of water, are taken by the hand lead. Soundings are taken while the vessel has headway on; the leadsman heaves the lead forward and gets the depth as the vessel passes, when the line is nearly perpendicular. A good leadsman in sufficiently high chains should get accurate soundings in 7 fathoms at 12 knots, 10 fathoms at 10 knots, and 15 fathoms at 7 knots; in depths greater than this, the speed must be reduced or other means used.
The lead line is made up of about 25 fathoms of three-quarter inch braided cotton twine marked in fathoms, with a wooden handle or toggle lashed into the line about two fathoms from the lead. Braided cotton twine best meets the requirement that the lead line should neither stretch nor shrink excessively as it is alternately wet and dried. The lead is securely attached to the line, and weights either seven or fourteen pounds.
Before marking, the line is well soaked, and then measured carefully and marked as follows from the lead:
These are known as "marks", and the numbers omitted are the "deeps". Soundings should be reported loud enough to reach the bridge, and in the exact phraseology indicated here:
Report Depth Corresponding With:
At night the leadsman should know the distance from his hand to the water's edge, and at each cast should subtract this distance from the mark in his hand for the true sounding.
The deep-sea lead, pronounced "dipsey", is briefly described here for historic interest. The lead weighs from 30 to 100 pounds and is attached to a line correspondingly heavy, marked every five fathoms - 1 knot at 5, 15, 25, 35, etc. fathoms; 2 knots at 20, 3 knots at 30, 4 knots at 40, etc. The bottom of the lead is hollowed out, and this hollow is filled with tallow before dropping. By thus "arming the lead" a sample of the bottom is brought up for comparison with the kind of bottom indicated on the chart.