132

NICHOLAS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE

provided for the purpose, and when the wire slackens up the lead will
have just touched bottom Apply the brake gradually and heave in.
Take the glass tube out of the container, taking care to keep it vertical,
and apply the tube to the wooden graduated scale from which the depth
in fathoms is read off. Lift the lead on board and examine the arming
and leave everything clear for taking the next sounding.

THE ECHO SOUNDING MACHINE

The Echo Sounding Machine is being rapidly developed and it is
assuredly a most effective aid to navigation The principle is simple
and depends upon the fact that sound travels in water at a known speed
and is reflected from the seabed in the same way that a sound wave in
air is reflected from hills and cliffs.

ECHO SOUNDING MACHINE—ADMIRALTY PATTERN.

Fig. 5.<—Recording Apparatus.
The application of the principle in practice calls for ingenuity and
specialised scientific knowledge in adapting electrical apparatus to
recording the very small interval of time taken by a sound wave to
travel down to the bottom of the sea and back again to the ship.