Page:The New International Encyclopædia 1st ed. v. 12.djvu/453

* LOFTTJS. 401 LOG. niid Susiaua (1857); and contributed to the Quarterly Journal of the (leoloyical Hociety and to the Journal of the Royal (Jcoyra/jhical Society. LOG (Swed. lof/y. Dan. loy, shijj's log; ulti- mately connected with Eng. loy, felled tree, and with Icel. liyyja, AS. licyan, Eng. lie). An ap- paratus for ascertaining the speed of a ship through the water. Logs are of two kinds, the old-type chip lor) and the patent or tuffrail log. The former consists of the log-chip, log-line, reel, and tiinc-ytass (14-second or 28-second). The chip consists of a piece of board about half an CHIP LOG. inch thick and cut in the shape of a quadrant of a circle of about six inches radius. Along its curved edge there is a groove filled with lead to ballast it and assist to keep it vertical, and near each angle there is a hole. In the holes at the ends of the circular arc are passed the ends of a small piece of rope which are knotted on the other side to keep them from pulling through. In the middle of this piece of rope is seized a wooden plug called a toggle. The end of the log-line is passed through the other hole in the chip and similarly knotted; on the log-line there is secured a piece of wood with a hole or socket in it for the toggle, and its distance from the chip is such that when the toggle is inserted the three lines hold the chip horizontal if the log-line is held vertically. The log-line consists of a small rope (untarred hemp usually) about one-quarter of an inch in diameter and 150 fathoms long, and is wound upon a reel fitted to turn easily upon its axis, which pi-otrudes beyond tlie reel to form handles by which it is held. The log-line is marked as follows: For a certain distance from the chip, usually about 20 fathoms, there are no marks; this is called the stray-line and must be long enough to let the chip get well clear of the eddies at the stern of the ship. The end of the stray-line is marked with a white rag. From that measure off the length for one knot (about 47 feet 3 inches for a 28second glass) and mark it with ci piece of cord worked into the lay of the rope and having one knot tied in its end: the second knot is marked with a similar piece of cord (usually hard-twisted fish-line) having two knots on it, and so on. Each knot is subdivided and marked at ever}' two-tenths with a small piece of cord without knots. The time-glasses are shaped like the conven- tion:il hour-glass and filled with a black sand which should be kept dry and free-running. To heave the log requires at least two persons, pref- erably three or more if the ship is going over four knots. One man holds the reel and another the glass; after throwing the log-chip (the toggle pressed firmly into the socket) over the stern he wait.s for the white rag at the end of the stray- line to pass over the ship's rail; if the reel does not work easily he assists it by pulling on the line, for no strain must come on the eliip, nor must the line lie slack in. the water. When the white rag crosses the rail he turns the glass (juickly or directs another man to do so — if there is a third — and when the sand is out the person holding the glass calls 'Up!' The line is grasped and held and the number of knots and tenths which have run out is noted. The pressure of the water against the chip when the line is held causes the toggle to pull out of the socket; the chip then lies flat on the water and the line is easily drawn in. When the line is properly marked, the glass in good ordef, and the heaving of the log carefully done, the result should not be in error more than two-tenths of a knot and should average less. But it must be remembered that thii- method only determines the speed at a jiarticular moment, and this may not be the average speed during the interval for which the speed is desired. A patent or taffrail log is very different in principle as well as in construction. It records the actual distance passed over by the ship and not the speed at any particular time. It con- sists of a register having three dials or other in- dicating devices, and appropriate gearing, a rota- tor which has a small body and blades like a BLISS PATENT LOG. propeller — but of greater proportional surface — and a log-line of hard braided untarred hemp connecting them. As the ship goes ahead, the rotator — which is a hundred yards astern — tends to twist the log-line; this in turn transmits the twisting motion to the register which, by suitable gearing, records the proper distance. In early types of tatlrail logs there was a tendency of the log-line to twist until the inertia of the gearing was overcome and then the latter would buzz around for two or three seconds. In recent types this is avoided by a little fly-wheel on the shaft to which the line is attached; this makes the