Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/795

L G L G 771 Fi 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. Air 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, vinless 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 liev. 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 (4130) 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 te 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 Yaughan Pendrcd, before the Society of Engineers, December C, 1869. The motions or disturbances im parted to the water by the body of the ship passing through it at 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 Fronde, F. R. S. , and Mr R. Edmund Fronde, see Brit. Assoc. Re}?., 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 uch 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 chamber 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 ba cut in the ship s bottom, by preference in the engine-room, large enough to allow a short cylinder (fig. 5) containing the screw 11 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 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 aflect- 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,"-nnd a. few of Berthon s, generally in conjunction with the old log-ship and line. (II. A. M.) LOGAN", JOHN (1748-88), a Scottish poet of some reputation, was born in 1748, and was son of George Logan, a farmer at Soutra, in East Lothian. Being destined for the church, he was in 1762 sent to study at the university of Edinburgh. After finishing his course, Logan was in 17G8-69 tutor at Ulbster to the well-known Sir John Sinclair, and in 1770 he edited some of the poems of his college friend MICHAEL BRUCE (q.v.). This publication was for the benefit of Brace s parents, who were in humble circumstances. In order to make up a volume he inserted some poems of his own, with some from other sources, and in his preface he stated that these could be easily distinguished without any names being attached. Of the seventeen pieces in the volume five were by Bruce, two by Bruce and Logan, eight by Logan, one by Sir James Foulis, and of one the authorship is unknown. One of the poems by Logan was " The Ode to the Cuckoo." In 1770 Logan was licensed as a preacher by the pres bytery of Edinburgh, and in 1771 was presented to the charge of South Leith, but was not inducted till 1773. In 1779 he delivered a course of lectures on the philosophy of history in St Mary s Chapel, Edinburgh. An analysis of these lectures was published in 1781 under the title of Elements of the Philosophy of History, and was followed by one of the lectures On the Manners and Govern ment of Asia, 1781. Logan was an active member of the committee of the General Assembly of the Church of Scotland appointed in 1775 to revise the "Translations and Paraphrases" drawn up in 1745 for use in private families, and to adapt them for public worship. The committee finished its.