Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/794

 770 LOG divide the degrees into one hundred parts, each to be called &quot; centesmes.&quot; The whole subject is treated as a new thing. It is stated by Purchas (1625) that Christopher Columbus (1492) deceived his crew with respect to the distance sailed from home, and that &quot; even the pilots did not know how far they had gone &quot; as they glided so smoothly with a continuous fair wind. Had any kind of log been hove, the ship s speed would have been publicly known. Mr Burnaby (Ancient Geography, p. 554) states that &quot;no ancient writer has preserved any account of the mode in which ancient navigators computed distance.&quot; Following such an authority and the quotations above, we may safely agree with the statement of Purchas that it was first used in 1607. Also we know that it did not become general till many years after. In one of our best works on navigation, printed in 1843, the log ia inaccurately described. If we are surprised that so many centuries passed, and that long voyages were made, after the discovery of the compass, without any means of measuring the distance sailed, we may be almost as much so at the diversity of opinion which prevailed among seamen with regard to the length of the log-line and the length of a mile. At the present day the principle upon which this log is arranged is easily understood. The mean degree of the meridian (see vol. x. p. 198) is assumed to be 69 09 statute miles, which gives 6080 feet to the mean nautical mile, an estimate sufficiently accurate for navigating upon any part of the sphere. The dis tances upon the log-line being marked by pieces of line placed between the strands and carrying the requisite number of knots, this has given the name of knot to the nautical mile. The line is marked to knots and half knots (a single knot) only ; the intermediate fractions are estimated. Two measurements are now in common use ; that in the British navy is 47 feet 3 inches of lino for each knot made per hour, which corresponds with a twenty-eight second glass, thus (28 x 6080) - 3600 = 47,288 feet ; in the mer chant service a knot is 50 feet 7 inches, which is the correct proportion to a mile with the half minute glass. When a ship is going more than five or six knots, a short glass is used,^ fourteen or fifteen seconds, then the indications by the line are doubled. The shorter measure was probably chosen in consequence of the custom in vogue till about 1833 of marking the run on the log-board, or book, in knots and fathoms (or sea furlongs) ; the fractions are now invariably entered as tenths. The whole length of line is 60 to 80 fathoms, according to the speed anticipated; 10 to 20 fathoms of which is allowed as stray line, that ths log-ship may be in a fair position, before the rag of bunting called the turn mark passes the hand. The line should be stretched and well wet before it is measured, and should be remeasured every day at sea. The inner end of the line is made fast to a light reel upon which it is wound. The &quot;log-ship&quot; (fig. 1) is a piece of wood about inch thick and the fourth part of a circle, having a radius of 5 or 6 inches, weighted with lead round the curve in order to keep it upright in the water, but not to sink it. Two holes are bored, about inches from the lower angles ; through one a short piece of line is passed and knotted; the other end of the line has a bone or hard peg spliced to it, Fi S- which is inserted in the other hole, thus forming a span by which it is attached to the log-line, and hangs square. When the log is usecl,&amp;gt; man holds the reel over his head the officer places the peg in the log-ship, and throws it well clear of the wake, then allows it to run the &quot; stray line&quot; off without assistance, steadying it just before the turn mark comes to hand ; as the mark passes he calls to his assistant with the glass to &quot;turn.&quot; As the sand runs pay out freely till the word &quot; stop &quot; is expected, then bring the line into a state of tension similar to what it was in when the turn mark passed. At the word &quot;stop&quot; nip the line instantly, count the nearest knots, and estimate the tenths. When the line is stopped the strain should cause the peg to draw from the log-ship, and it can easily be hauled in. In ships of war it is hove every hour. The value of the operation depends, of course, entirely upon the care bestowed. Ground-Log. In large rivers, such as Rio de la Plata, where a strong current runs, and shoals are found out of the sight of land, a lead of four or five pounds weight is used instead of the log-ship ; the lead rests on the bottom, the line and sand-glass being used in a manner similar to that above described. This is called the ground-log, and indicates the speed at which the ship is passing over the ground, irrespective of currents or tides ; it will show also the lateral effect of current as it is hauled in; this is the only log which can do so. The sand-glasses are very primitive contrivances for measuring the requisite number of seconds ; they are much affected by damp and change of temperature, and no reliance can be placed on their accuracy. In 1868 a timepiece sounding a gong at the required intervals was devised by the late Admiral Sir Walter Taiieton, and was tried on board some of Her Majesty s ships, but failed after a short time from clamp or other causes. The writer of this article was then attempting to produce a log-gong, but abandoned it on being told that they could be obtained below his estimated cost. Screw Logs. In 1725 Henry de Saumarez described a machine which was to supersede the ordinary log. This was on the prin ciple of the screw, having vanes which caused it to revolve and communicate a rotary motion to a piece of rope; this most probably went inboard to clockwork; hence the failure. Mr Smeaton made many experiments about 1751 ; he found the results very irregular, especially at high velocities, just as the writer of this article did with one of Massey s flies and a line or wire attached to a spindle, supported by large friction rollers inboard ; both experiments were dead failures on account of the friction. In 1773 two screw logs were tried on board H.M. ship &quot;Racehorse&quot; during a short voyage to the Polar Sea ; each was made to communicate motion to a counter inboard. In 1792 Mr Cower took out a patent for a screw log. None of these experiments were sufficiently successful to gain the confidence of seamen. We see that the principle was not new in 1834 when Mr Massey patented a screw log, which has been so generally adopted that it de serves special notice and description. Though Massey took out other patents, and others have followed with modifications, the principle of all is the same, and likely to remain in use with the &quot;common log &quot; for many years to come. Massey fitted his log to the stern- post of a few vessels, a vertical spindle conveying the rotations of the fly to a register in the cabin above, but it did not answer. The log of 1836 which came into general use is represented in fig. 2. It con sists of two parts united by 2 or 3 feet of rope. The &quot;fly &quot; consists of a hollow copper cylinder about 9 or 10 inches long with four fins or blades placed at a given angle, causing it to rotate once in a certain distance. The rope is attached to the fly and to a spindle which freely revolves in a brass box ; an endless screw acting upon a system of wheel-work records the fractions of a mile on one dial, units upon a second, and tens up to one hundred on the third, on the same principle with the index of a gas meter. The last patent was for the &quot;frictionless log &quot; shown in fig. 3, which is similar to the former ex cept that, by dispensing with the piece of rope and part of the heavy box, it is much more compact and less liable to foul, an accident to which all logs when towed after a ship are very liable. Walker s harpoon log is very similar