This is the principle of the log : in practice the log is a flat piece of wood, generally of the figure of a quadrant, loaded with lead on its arc, to make it float upright ; to this is attached a line about 150 fathoms long, divided into equal lengths by little pieces 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. All the line between the log and the rag is called the stray line, and is of course omitted from the account. When the log is thrown into the sea, which ie done from the lee quarter of the vessel, the log-line, by the help of a reel on which it is wound, is immediately veered out, at least as fast as 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. There are various ways of dividing the line, the most usual of which is to place the knots at distances of 50 feet from each other ; now as 120 times half a minute make an hour, and 120 times 50 feet make almost a geographical mile, so many knots will run from' the reel in an expeii ment as the vessel sails miles in the hour ; from this comes the expres sion of a vessel's sailing so many knots an hour—meaning miles. Fifty one feet would be more accurately 120th part of a mile than 50 feet ; but it is found practically that the ship's way is always a little more than that given by the log, arising from the circumstance that the line is unavoidably pulled in some degree, and the log is consequently ust a fixed point ; it is moreover alder to have a ship behind the re '-ring than before lt, which induces many commander's to shorten the distances between the knots to forty-eight and even forty-five feet. Whatever distance be taken, it is found convenient to
subdivide into ten parte for decimals of a mile. Careful commanders retneeetire the )(TIM° frequently, to ascertain if it varies from its original length. In case of an alteration they apply a correction to the rate found by a commonin the rule of the length which the commander is to the real interval, so is the apparent rate to the true rate. A similar correction is required if the half minute glass is found to be wrong.
In the beet regulated vessels the log is hove every hour ; mad in calculating the chip's going it is supposed that the rate has not varied between the intervals of heaving; but if the wind has sensibly varied, or more or lees sail Iran' been pet during the time, then an allowance is made according to the discretion of the person who keeps the account.
A tow years ago a very curious lag was invented by Mr. Ilookey, which though ingenious was too complex to come into general nee; its object was to afford as great a resistance as possible to the pull of the line, and at the same time to be easily drawn back to the ship when its work was done. This log is shaped like a fish, and the line is in its mouth.
A more practically useful suggestion of Mr. Harkey was to soak the Lne in a mixture of three parts linseed oil and one part fish nil, which prevented its shrinking ; a matter of no small import. ance when it is considered that a new line without preparation will lass 60 or 60 feet of its length by contraction when wetted.
Of late yews Ilamey'n patent log has been used with great benefit to navigation, as the time being noted when it is put overboard, it may be allowed to tow a-stern for hours, at the expiration of which the number of revolutions of a vane attached to it, may be read off from a self-registering dial, and compared with the time, from which is deduced the chip's velocity. lta form is too well known to need further description.