The log-line is secured to this span and consists of two parts.
Such a "ground-log" indicates the actual speed over the ground, and in addition, when the log-line is being hauled in, it will show the real course the ship is making over the ground.
Sometimes a 30-second glass is used instead of a 28-second one, and the intervals between the knots on the log-line are then made 50 ft.
For speeds over six knots a 14-second glass is employed, and the speed indicated by the log-line is doubled.
To ascertain the ship's speed by the common log four articles are necessary - a log-ship or log-chip, log-reel, log-line and log-glass.
When the log-line is quickly nipped, the knots counted, and the intermediate portion estimated.