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.