LOG, MARITIME, an appliance for measuring the speed of a ship through the water. The oldest form of log of a practical nature was that known as the "common log." The outfit consisted of a log-ship or log-chip, log-reel, log-line and log-glass. The log ship was a wooden quadrant about 4 inch thick with a radius of about 5 or 6 inches, its circumference being weighted with lead to keep it upright in the sea and to retard its passage through the water. One end of a short length of thin line was passed through a hole at one of the lower corners of the log-ship and secured by a knot on the opposite side. To the other end was spliced another short length terminating in a hard wooden peg which could be inserted in a similar hole in the opposite corner. The holes were so placed that the log-ship would hang square from the span thus formed.
The log-line led from this span inboard to the ship. The first length between the log-ship when floating in the sea and the ship was known as the "stray line." It varied from io to 20 fathoms according to the distance necessary to veer the log-ship clear of the disturbance of the ship's wake before starting observations. The place where its inboard end joined the remainder of the log line was marked by a piece of bunting, and the line from here towards the end in the ship was marked and measured in "knots" composed of pieces of cord worked in between the strands. The distances between the knots bore the same relation to a nautical mile (6,o8o ft.) as 28 sec. to an hour (3,60o sec.); i.e., they were placed 47 ft. 3 in. apart. The end of the first interval, counting from the bunting, was marked with a piece of leather, the second by a cord with two knots, tie third by one with three knots, and so on. Half-way between each length was a cord with one knot. It followed, therefore, that if, say, five knots of the line ran out in 28 sec. by the sand glass, the ship had gone 5 X47+ ft. in that time, i.e., was moving at a rate of five knots (five nautical miles per hour). For speeds over 6 knots, a 14-second glass was employed and the speed indicated by the log-line was doubled. The form of marking adopted enabled the log to be read when taking readings at night.
The actual process of "heaving the log" was as follows : The log-line was prepared by well soaking and stretching it before marking. It was then wound evenly on the log-reel, to which its inner end was securely attached. One man held the log-reel over his head, the officer (in the navy, usually the midshipman of the watch) placed the peg in the log-ship which he then threw over board clear and to windward of the ship, allowing the line to run out freely. When the bunting at the end of the stray line passed his hand, he called to his assistant to turn the sand-glass. When all the sand had run out, the assistant called "stop," when the log-line was quickly nipped and the knots counted, any extra por tion intermediate between two markings being estimated. The strain brought on the little span next the log-ship caused the peg to be withdrawn from its hole, when the log-ship turned edge on, thus facilitating hauling it in. Normally the log was hove every hour,
and under favourable conditions the common log gave very fairly accurate results. It has, however, been almost entirely superseded by automatic or "continuous" logs registering inboard on dials.
One of the earliest of these logs was invented by Edward Massey in 18o2 ; this log was in general use in 1836 and continued so until 1861. An improved log was produced by Alexander Bain in 1846, and in 1861 Thomas Walker's "harpoon" or frictionless log was introduced. In all these the dials registering the distance travelled were close to the screw which actuated the log, i.e., they towed overboard and the instrument had to be hauled in before it could be read. This inconvenience was overcome by Walker, when in 1878, he introduced his "cherub" log. Walker's "cherub" con sisted of a dial secured to the taffrail of the ship, a specially plaited line of 4o to 6o fathoms in length and a screw-like rotator. To regulate the rate of spin, a brass fly-wheel, known as a "gover nor," was added in the later patterns. The "cherub" log gave excellent results up to a speed of about 14 knots ; above that the rotator was inclined to jump out of the water, so to overcome this defect the "trident" log was introduced. This log was generally similar in appearance to the "cherub," but the line was 65 fathoms, and certain improvements were made to the mechanical parts of the counter to enable it to bear the strain of higher speeds. A further development was an electrical arrangement transmitting the readings of the counter to a dial in the chart-house.
The transmitting apparatus consists essentially of a tube pro jecting through the bottom of the ship and turned so as to face forward. Water is forced through this tube by the forward move ment of the ship; the flow operates a small propeller which actu ates the transmitting mechanism. The water flows out through another opening facing aft. The small propeller operates a com mutator which transmits electric signals to the distant recorders. It also drives a magnetic generator which generates currents of a voltage proportional to the speed. The commutator operates the dial indicating the distance travelled, while the generator excites a voltmeter graduated in knots. Arrangements are made to adjust the mechanism to suit and record accurately for an individual ship.
(E. A.)