LOG. The ordinary log for ascertaining the speed of a ship consists of four parts, viz., the log-glass, log-line, log-reel, and log-ship. The word log may have been derived from the fact that a piece of wood was thrown overboard, to lie as a log in a fixed position, motionless; now the same name is applied to many contrivances and ingenious inventions for indicating directly, or for registering, the ship's progress through the water. '''
Though such information now appears to be so essential, nay, imperatively necessary to the safe conduct of a ship, it is a fact that no such simple means as the log and line was devised before the 17th century, or the subject even thought of theoretically before 1570. At least nothing can be found in ancient writings, or even in the works professedly treating upon navigation, till after 1620, while, on the contrary, various passages occur from which we may fairly infer that there was nothing better at the command of the mariner than a rough unassisted estimate. The work of Martin Cortes (Seville, 1556), after giving much valuable information for that day, including a description and use of the cross-staff, astrolabe, &c, a table of the sun's declination, with much else, makes no other reference to the ship's motion through the water than this,the pilot must estimate the distance, making allowance for the effects of winds and currents, every day, and as the estimation " is imperfect, especially in a long voyage and long time, it is convenient that he should rectify his position by the corresponding position of the heavens." Mr J. Tapp, who published a translation and improved edition of Martin Cortes fifty-three years after (1609), made no alteration in that part of the work.
In 1578 William Bourne published Inventions and Devices. There are one hundred and thirteen subjects treated of, many of them highly interesting, as they contain the crude germ of useful inventions. The twenty-first device is a close approach to Massey's self-registering log, which was found so useful two hundred and sixty years later. The credit of the device is ascribed to Humfray Cole; the probable date is 1570. The proposal was to have a " little small close boat" with a wheel, or wheels, and an axletree, to turn clock-work in the little boat, with dials and pointers to indicate respectively fathoms, leagues, scores of leagues, and hundreds of leagues. If a small screw rotator had been used instead of a wheel, this might have been a great success. It was only a suggestion, perhaps untried; and in common with seamen and writers about that time the author allows only 5000 feet to a mile. Edward Wright's Certain Errors in Navigation detected and corrected (1610) gives much new and useful information, but the nearest allusion to the ship's speed is in the part translated from the Spanish of Roderigo Samorano, under the head of finding the ship's place on the chart, called the " point of imagination." " This point doth presuppose the knowledge of two things : to wit, the rhumb by which we have sailed, and that is known by the compass, and the leagues which we have run; and this hath no certainty, but is a little more or less than a good mariner according to his imagination supposeth that he hath sailed ; whereof the said point took its name." In 1624 an edition of Gunter by Edward Weaver, after much valuable geometric information, proposes at chap, vi., in a long rambling manner, that an account should be kept of the ship's way. "The way that a ship maketh may be known to an old seaman by experience, by others it may be found," as he recommends, with the log-line or by known marks on the ship's side, bearing the proportion to a league or mile, that a certain number of seconds do to an hour. So far good ; but he reckons a mile as 5866 feet (214 too little), and states that seamen count in paces of 5 feet each, and 1000 to a mile, i.e., only 5000 feet. He also proposes to divide the degrees into one hundred parts, each to be called " centesmes." 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 " even the pilots did not know how far they had gone" 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 "no ancient writer has preserved any account of the mode in which ancient navigators computed distance." 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 is 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 line 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 the 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.
thus forming a span
The "log-ship" (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 1J 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, which is inserted in the other hole, by which it is attached to the log-line, and hangs square,
When the log is used, a 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 " stray line" 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 "turn." As the sand runs pay out freely till the word " stop " 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 " stop " 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 Tarleton, and was tried on board some of Her Majesty's ships, but failed after a short time from damp 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 "Racehorse" during a short voyage to the Polar Sea ; each was made to communicate motion to a counter inboard. In 1792 Mr Gower 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 '' common, log " 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 1
feet of rope. The "fly" 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 "frictionless log " shown in fig. 3, rjimrfU which is similar to the former ex- >==««fcJ-JJ 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 to the last of Massey's, but has a plate at the back in the shape of a harpoon to prevent the upper part from revolving. This log is now supplied to Her Majesty's ships. The fins or blades which cause the rotation in each of the logs above described are flat pieces of brass (not portions of a screw) soldered to a cylinder, which is hollow in order to diminish the tendency to sink when going slowly; but if the log be left overboard when the ship stops, the tow-line will allow it to sink about 100 feet, the pressure of water will then fill it, and there is no means provided for getting the water out. Screw logs will also at low speeds hang obliquely and be useless. Mr Friend tried a log with paddles protruding from a brass box instead of using a screw ; but the plan was not adopted. However accurate the registering logs may be, an hourly log cannot be dispensed with, unless the ship be on one course during the whole twenty-four hours, or her speed be uniform; even then the old log and line should not be neglected. Both Massey and Walker are now trying logs the rotators of which are towed, while the dials for registering are on the ship's stern.
Pressure Log.In 1849 the Rev. E. L. Berthon patented a log (fig. 4), indicating the speed of the ship by means of the pressure of water due to the velocity acting upon a tube about j of an inch diameter in the clear, closed at the end, protruding some 8 inches below the ship's bottom, with an aperture of about an \ of an inch in, diameter in the front side, near the closed end. A vane was used to turn the aperture in the direction of the ship's progress (course and leeway combined). At the upper end of the the same pipe a pointer indi-cated the amount of leeway. To take into account the effect which change of draught would produce, another pipe was used having the aperture in a neutral direction (41°30) with regard to the ship's progress, so that the water was neither forced in nor drawn out. The two pipes communicated with air-vessels, which were allowed to be about half full of water; thence two flexible tubes conveyed the pressure to the ends of an inverted siphon partly filled with mercury, one leg of which forms a glass index tube, a graduated scale being placed behind it, calculated upon the principle that the pressure will increase according to the square of the velocity. As the specific gravity of mercury is so great, the scale even up to 16 knots is brought within a convenient com-pass; and it can be hung in gimbals (as a barometer) in any part of the ship. The leeway indicator in more recent fittings has been abandoned. The writer of this article first saw it in one of the Jersey packets, when she was steaming about 13 knots ; it appeared to be very sensitive, and he was strongly impressed in its favour. For details respecting this log see paper by Vaughan Pendred, before the Society of Engineers, December 6, 1869.
The motions or disturbances im-parted to the water by the body of the ship passing through it at c high velocity must vitiate in a great degree all attempts to mea-sure the speed by instruments placed near the hull of the ship, and under varying circumstances of draught, speed, and foulness of bottom. For the results of ex-periments and opinions on this point, by the late William Froude, F.R.S., and Mr R. Edmund Froude, see Brit. Assoc. Rep., 1874, p. 225, and 1879, p. 210.
Electric Log.In the chrono-logical order in which we have taken various descriptions of log, the last deserving notice is Kelway's "electric log," the only such log known to the public. Its chief feature is the making and breaking of an electric circuit by means of a screw revolving in the water and an electric battery connected with the step motion indicator. One of the difficulties to be overcome was that of securing a thamber wherein to form the electric contacts, which should remain watertight under the pressure due to its depth below the surface of the sea, particularly in the event of the ship stopping and suffering it to sink when being towed with 50 fathoms of line. Mr Kelway now believes that he has overcome that difficulty, and his log has been tried on board several of Her Majesty's ships at Portsmouth, with satisfactory results, a screw similar to Massey's being towed, while in electric connexion with a dial on board.
What is considered by Mr Kelway to be an improved application of the principle is now (1882) on view in the International Exhibi-tion at the Crystal Palace. It is intended that a hole should be cut in the ship's bottom, by preference in the engine-room, large enough to allow a short cylinder (fig. 5) containing the screw R to pass down below the ship's bottom. The cylinder is open in a fore-and-aft line and attached to a cage H, which is drawn up or lowered by means of a large screw G working through a stuffing box F. The iron box D containing the cage is 4 feet in height, made in three parts ; the lower part (high enough to receive a sluice valve C) is to be bolted to the ship's bottom, and must, with the rest of the box, be nearly as strong; the central part is secured to the valve box and covered by a lid E, there being space enough above the sluice valve for the cage and screw.
To place the log, let the sluice valve C be lightly closed ; open two small taps to let the water out of the box and to prove that the valve is acting. Open the lid, run the lowering screw through the cage, place it, secure the lid, open the sluice valve, and lower the rotator to the desired distance. The blades of the rotator are por-tions of a true screw. An endless screw on the spindle of the rotator communicates the revolutions to a vertical spindle M, which moves a train of wheels in a watertight box N ; the last of these wheels revolves once in a mile, and on the same spindle is a wheel having eight ratchet teeth, which by moving a lever complete an electric current, which passes by the wire 0 to a dial placed in any part of the ship, sounding a bell and causing one hand of the dial to make a step and mark an eighth; one revolution indicates a mile, and other dials carry the register up to 100 miles. This form of electric log has, however, the disadvantage pointed out as affect-ing the Berthon or any log placed under the hull of a ship.
The electric towing log (by Kelway) promises to show continuously on board the ship what she is doing, while keeping a record of what has been done. A rating table would be at the dial, in any part of the ship; or several dials could be worked by the same electric current. It will be exposed to the danger of fouling sea-weed, &c, as other towing logs are.
The logs now generally used are Massey's, Walker's, and a few of Berthon's, generally in conjunction with the old log-ship and line. (H. A. M.)
The above article was written by Captain H. A. Moriarty, R.N., C.B.