LOG AND LOG'LINE. This is the appa ratus by which the velocity of a 'ship's motion through the water is measured. The log is a fiat piece of wood, loaded with lead at one of its edges to make it float upright ; and to this is attached a line about 150 fathoms long, divided into equal lengths by little piece's of knotted twine rove into it. These divisions begin about twenty or thirty yards from the log, where a piece of red rag is usually fastened, in order to show"the place readily. From the lee quarter of the vessel the log is thrown into the sea, where it is Supposed to remain stationary during the operation, and the line is veered out at least as fast at the ship sails. As soon as the red rag leaves the reel, a half minute glass is turned, and, when the sand is all run down, the reel is stopped. Then, by measuring the quantity of line run out, the distance sailed by the vessel in half a minute is known, and by calcUlation its rate of going per hour. The usual way of dividing the End
is to place the knots at distances of fifty feet frObi each other. Now, as 120 times half a minute Make an hour, and 120 times fifty feet Make almost a 'geographical mile, sd many khott will Hui from the reel in one experiment At the Vessel sails miles in the hour; from this Comet the expression of a vessel's sailing to many knots an hourómeaning miles. Mr. Berthen's log, patented in 1850, registers the speed of Ships out at sea by the height of a column. of water raised by the resistance. There are two tubes which project beyond the bottom of the vessel ; those tubes have small apertures through which the water enters, the height of ascent being greater as the speed of the vessel increases. There are various minor contrivances to measure the steed by the indications obtained.