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Page 40
The half-minute glass used on board the Young America, held by the
quartermaster, was like an hour glass, and contained just sand enough to
pass through the hole in the neck in thirty seconds. The log-line was
one hundred and fifty fathoms in length, and was wound on a reel, which
turned very easily, so that the resistance of the chip to the water
would unwind it. The log-line is divided into certain spaces called
knots, the length of each of which is the same fractional part of a mile
that a half minute is of an hour. If there be sixty-one hundred and
twenty feet in a nautical mile, or the sixtieth part of a degree of a
great circle, which is not far from accurate, and the ship be going ten
knots an hour, she will run sixty-one thousand two hundred feet in an
hour. If the chip were thrown overboard at eight o'clock, and the line
were long enough, the ship would have run out sixty-one thousand two
hundred feet, or ten miles, at nine o'clock, or in one hour. In one
minute she would run one sixtieth of sixty-one thousand two hundred
feet, which is ten hundred and twenty feet; in half a minute, five
hundred and ten feet.
The half-minute glass is the measure of time generally used in heaving
the log. While the sand is dropping through, the line runs out five
hundred and ten feet, the ship going ten knots an hour being the basis
of the calculation. One knot, therefore, will be fifty-one feet. If the
line pays out five hundred and ten feet in thirty seconds, by the glass,
the ship is going ten knots an hour. If it pays out four hundred and
eight feet in half a minute, or eight hundred and sixteen feet in a
minute, she will pay out a mile in as many minutes as eight hundred and
sixteen feet is contained in sixty-one hundred and twenty feet, which is
seven and a half minutes. Then the ship goes a mile in seven and a half
minutes, or eight miles an hour.
A knot on the log-line is therefore invariably fifty-one feet; and the
number of knots of the line run out in half a minute indicates also the
ship's speed per hour, for fifty-one feet is the same part of a nautical
mile that half a minute is of an hour. The calculations are given
without allowances, merely to show the principle; and both the glass and
the line are modified in practice.
On board the Young America, ten fathoms were allowed for "stray line;"
this length of line being permitted to run out before the measuring
commenced, in order to get the chip clear of thee eddies in the wake of
the ship. The ten fathoms were indicated by a white rag, drawn through
the line; and when the officer paying out comes to this mark, he orders
the quartermaster to turn the glass, and the operation actually begins.
At every fifty-one feet (or forty-seven and six tenths, making the
allowances) there is a mark--a bit of leather, or two or more knots. The
instant the sands have all run through the glass, the quartermaster
says, "Up," and the officer notes the mark to which the line has run
out. Half and quarter knots are indicated on the line.
"Now, quartermaster, mind your eye. When the officer of the deck says,
'Turn,' you repeat the word after him, to show that you are alive,"
continued Peaks.
"Ready!" said Gordon.
"Ready!" replied Smith.
The lieutenant threw the chip into the water, and when the stray line
had run off, he gave the word to turn the glass.
"Turn!" repeated Smith.
Gordon eased off the log-line, so that nothing should prevent it from
running easily.
"Up!" shouted Smith; and Gordon stopped the line.
"Very well," added Peaks. "What's the mark?"
"Ten and a quarter," replied the officer.
"That sounds more like it. I knew this ship was going more than seven
knots. You see, young gentlemen, you can't catch flies and tend the
log-line at the same time. Now, you may try it over again."
The experiment was repeated, with the same result. Other officers and
seamen were called to the quarter-deck, and the training in heaving the
log continued, until a reasonable degree of proficiency was attained.
"Land ho!" cried the lookout on the top-gallant forecastle, at about
eleven o'clock in the forenoon.
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