A Treatise On The Principles And Practice Of Harbour Engineering

44 HARBOUR ENGINEERING. depth can be read directly therefrom. This method is perfectly satisfactory when performed from a statiouary base, as, for instance, when the operator is standing on a quay wall or on a boat which is moored, and in this way it is, of course, only applicable to single dips. When it is desired to take a series of dips along a given line in any direction, the base, if afloat, must obviously be movable ; and while a boat, no doubt, may be alternately moved and moored so as to fulfil the condition stated above, yet the process would be slow and tedious. It is evidently preferable to adopt some method of taking soundings in close and un- Tag- Copper Link. %2 Steel ^^Tag Tag. Lead Fig. 40. — Wire Sounding-line. Brass rLinks Copper Link. Brass Links nCopper Link. Lead Fig. 41. — Chain Sounding-line. interrupted sequence while the boat is in continuous motion.. Lowering a pole or line under these circumstances would lead to inaccurate readings, as, by the time the bottom was reached, the travel of the boat would have produced considérable inclination in the instrument, so that it was no longer ver- tical. Seamen get over this difficulty by throwing or heaving the lead, which weights the line, some distance ahead of the boat, giving it time to reaoh the bottom before the boat passes perpendicu- larly over it. The reading is then taken at the moment of verticality, as near as can be judged. The method is somewhat rough and crude, but, except in deep water, it gives fairly reliable results. The hand-lead, ordinarily used for the purpose, has alength of line not exceeding 30 fathoms, which is more than ample for any engineering re- quirements. The length of line for pur- poses of harbour soundings need hardly exceed 50 or 60 feet, and very often much shorter lengths than this will serve. The line, if of hemp or wire, is graduated by tags of different texture, shape, and colour, so as to be identified by night as well as by day (fig. 40). Strips of woollen material, cotton, leather, serge, and even string, are utilised to give variation. If a chain line be used, copper-stamped links are substituted for the tags (fig. 41). A Chain is preferable to a hemp line on account of the excessive shrinkage of the latter, amounting, when new, to as much as 5 per cent, during the course of a day’s work. New lines should, in fact, be avoided for this reason, and all lines should be well wetted just before use. Finally, they should be tested frequently—before and after each line of soundings, if possible—by some standard length, which, for all practical purposes, may be marked on the boat itself.