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CHAPTER III. OF THE LOG-LINE AND HALF-MINUTE GLASS. Various methods
have been proposed for measuring J;he rate at -which a ship sails;
but that most ia use is by the Log and Half-Minute Glass. The Log
is a flat piece of thin board, of a sectoral or quad- rantal form,
Fig. 220, Kate 25, loaded, on the circular side, with lead
sufficient to make it swim upright in the water. To this is
fastened a line, about 150 fathoms long, called the log-line, which
is divided into certain spaces called knots, and is wound on a
reel, Fig. 219, which turns very easily. The Half-Minute Glass is
of the same form as an Hour-Glass, Fig. 218, and containing such a
quantity of sand as will run through the hole in its neck in lialf
a minute of time, or twenty-eight or fourteen seconds. The making
of the experiment to find the Telocity of the sliip, is called
heaving tJic log, which is thus performed: ?One man holds the reel,
and another the half-minute glass; an officer of the watcli throws
the log over the ship's stem, on the lee side, and when he observes
the stray line is run off (which is about ten fathoms, this
distance being usually allowed to carry the log out of the eddy of
the ship's wake), and the first mark (which is generally a whito
rag) is gone off, he cries, Turn; the glass-holder answers, Tarn;
and watching the glass, the moment it is run out, says, Up. The
reel being immediately stopped, the last mark run off shows the
number of knots, and the distance of that mark from the reel is
estimated in fathoms. Then the knots and fathoms together show the
distance the ship has run the preceding hour, if the wind has been
constant. But if the wind has not been the same during the whole
hour, or interval of time between heaving the log, or if there has
been more sail set or handed, a ...
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