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SURVEYING, MARINE AND SUBMARINE.
45
A seaman’s lead is usually an octagonal bar, weighing from 8 to 10 Ibs.
For engineering work, a flat disc weighing about 5 Ibs. is more suitable, as
there is less tendency for a lead of this shape to sink into mud or other soft
material. If, however, it be desired to penetrate through the mud to firm
ground, a bar or ball must be employed.
For soundings in shallow water, in depths, say, not exceeding 20 or 25 feet, a
pols is sometimes used. As regards convenience in handling, white pine, which
is light, forms the best material from which to make it. The pole is either
circular in section, or oblong, about 2 inches by 3 inches, with hollowed faces,
painted and graduated in feet and quarter-feet ; sometimes in feet and inches.
It should be shod with a flat-bottomed shoe for the reason stated above, and
the weight of the shoe should be just sufficient to assist in sinking the pole
to the bottom, and no more. The manipulation of a long pole, however, is by
no means an easy matter, and it is only or mainly used for minor purposes.
The drawback attaching to all these appliances—line, chain, and pole—is
their liability to miss some prominent protuberance in the bottom due to
the isolated nature of the dips and the distance which lies between them.
Except in perfectly still water, it is necessary to keep the boat moving at a
certain rate in order to steer it and prevent it from being deflected out of its
course by the current. It is difficult, therefore, under ordinary circumstances,
to take soundings with the lead line at intervals of less than 10 feet. And
a good deal may lie hidden in 10 feet. As a matter of fact, soundings are
frequently taken at much greater distances. Furthermore, there is the
effect of wave motion, which interferes very materially with the accuracy of
the readings.
For these reasons, and for others which it is unnecessary to enumerate, a
more complété and reliable system of recording depths in connection with
harbour work is highly desirable, and a number of attempts have been
made to supply apparatus which will conform with the requirements of the
case. Chemical and electrical agencies have been proposed, and tested with
varying, but generally unsatisfactory, results. They are too sensitive in
action and too delicate in adjustment for use in exposed situations amid
unstable surroundings. Whatever possibilities they may contain, at any-
rate they have not yet been put into a working form, and mechanical ap-
pliances still seem to supply the only practicable means of dealing with the
problem.
For sounding work in estuaries, harbours, and coastal inlets generally, the
most serviceable and efficient machine with which the author is acquainted
is one designed and patented by Mr Fielden Sutcliffe of Liverpool. Having
had occasion to use it many times, the author feels in a position to speak
authoritatively on its capabilities, and he has no hesitation in testifying to
its value. The following is a description of the machine.
Suteliffe’s Sounding Apparatus.—The apparatus illustrated in ligs.
42, 43, consiste of three parts: the Sounding Machine (shown fitted up on
a boat), the Horizontal Distance Measurer, and the Section Plotter.