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.