Observations of Temperature.—The temperature of the water is usually taken at intervals of ma fathoms to a depth of I,000 fathoms, and at closer intervals in the first ioo fathoms. If a second wire machine is available, the observations may be made from aft whilst the sounding is being taken forward. A 30-lb. sinker is attached to the end of the wire, and the register ing thermometers are secured to the wire by the metal clips at the back of the cases, at the required intervals. To avoid heavy loss, no more than four thermometers should be on the wire at one time. When sounding a thermometer is usually attached to the line a short distance above the lead.
Submarine Sentry.—The primary object of the machine called the "submarine sentry" is to supply an automatic warning of shallow water; it has been instrumental in discovering many unsuspected banks in imperfectly surveyed waters. By means of a single stout wire the sinker, an inverted kite, called the "sentry," can be towed steadily for any length of time, at any required vertical depth to 4o fathoms; should it strike the bottom, it will free itself and rise to the surface, simultaneously sounding an alarm gong. The vertical depth at which the sentry sets itself when a given length of wire is paid out is not changed by any variation of speed between five and 13 knots.
The point of the catch C, passing through a thimble M in the short leg of the sling, is slipped into the hole at the top of trigger T, which is hinged at K and kept in its place by the spring S attached to the hook H. On the trigger striking the bottom the catch is released, the short leg of the sling slips off, and the sentry, which then rises to the surface, is left towing by the long leg. The winch is fitted with two handles for heaving in the wire ; one gives great power and slow speed, and the other, acting on the drum spindle direct, winds in quickly. The wire supplied with the machine has a steady breaking strain of about i,00c) lb. Echo Sounding.—This new method depends upon the prin ciple that any sound originating at or near the surface travels through the water at a known speed, and after striking any large object or the bottom of the sea is reflected back to the surface, the time being a matter for accurate measurement.
The time necessary for obtaining a sounding is almost infini tesimal, the sounding in extreme depths taking little more time than one in the shallowest water. The speed of the vessel is maintained throughout the soundings, the soundings themselves may be accurately taken at extremely short intervals and bad weather does not affect the work. See also CHART and SOUND.
See Echo Sounding published by H.M. Stationery Office. (J. A. ED.)