LOG. A machine used to measure the rate of a ship's velooity through the water. It is a piece of thin board, forming the quadrant of a circle of about six inches radius, and balanced by a small plate of lead, nailed on the circular part, so as to swim perpendicularly in the water, with the greater part immersed. The log-line is fastened to the to by means of two legs, one of which is knotted, through a bole at one corner, while the other is attached to a pin, fixed in a hole at the other corner, so as to draw out occasion ally. The log-line, being divided into certain spaces, which are in proportion to an equal number of geographical miles, as a half or quarter minute is to an hour of time, is wound about a reel. The whole is employed to measure the ship's headway, in the following manner :— The reel being held by one man, and the half-minute glass by another, the mate of the watch fixes the pin, and throws the log over the stern, which, swimming per pendicularly, feels an immediate resist ance, and is considered as fixed, the line being slackened over the stern. to pre
vent thepin coming out. The knots are measured from a mark on the line, at the distance of 12 or 15 fathoms from the log. The glass is, therefore, turned at the in stant that the mark passes over the stern; and, as soon as the sand in the glass has run out, the line is stopped. If the glass runs 30 seconds, the distance between the knots should be 50 feet. When it runs more or less, it should, therefore, be corrected by the following analogy : As 30 is to 50, so is the number of seconds of the glass to the distance between the knots upon the line.