134 NICHQLLS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE
the mam switch, situated on the underside of the receiving box ana
puts on the telephone receivers. A faint tapping will be heard but this
is disregarded. He now works his hand wheel, moving up a foot at a
time, untiKhe tapping suddenly becomes louder, and reads o££ the depth
directly from the scale. The time taken by the sound wave to travel
to the bottom and back is converted by the recording apparatus into
echo feet or fathoms. Figure 5 shows the recording apparatus with the
door open, and Figure 6 gives a diagrammatic idea of the same apparatus
in a convenient place on the bridge, with the cable leading down to the
transmitter where a hammer is operated by a solenoid; the sound wave
thus set in motion travels to the bottom and up again, the sound being
picked up by the hydrophone and passed on through the return cable to
the head telephone of the observer.
An additional instrument is now being attached to the apparatus,
which produces automatically a trace of the contour of the seabed and
thus exhibits a continuous picture of what the vessel is passing over and
relieves the officer of the duty of listening-in.
THE COMMON LOO
The common log consisted of a reel on which a line, marked ofi
into equal lengths to represent rniniatu:;jT^tical miles and called
"knots," was wound, a log chip and a 1 xjS^\/^^4i ;|^ ^e ^°§
chip was a triangular shaped piece of C\ ^^|. jjjfe|,i!'p^ edge so
that it would float vertical, apex npw8^'^<:^^Ki~^^^^S'h& log line
was secured to the three corners of the ^» \J \ji$ /'AH^8 °* a three-
legged bridle. -~-_-~-'^V ia~
The operation of "heaving" the log was as follows. One man held
the reel, another man the sandglass and the officer threw the log chip
into the water and paid out the line. The log chip remained stationary
in the water astern and the ship sailed away from it. When a certain
length of line, called "stray line" to allow the log chip to get clear of
the eddy water in the wake, passed over the rail the officer called out
"torn" and the man turned the glass. As soon as the glass ran out he
called out "stop"; the officer held the line and noted the nearest mark
and so got the speed the ship was going at the time.
The Principle in dividing off the log line was to make the distance
between the knots on the line bear the same proportion to a nautical
mfle (6080 feet) as the seconds of the glass bore to the seconds in an
hour (3600).