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.