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8 THE MARINE RECORD. May 2, Igor Haance ce anreecraaeceaerrecreennenercnr aaa SS SN AL TL LLL TS THE LAW OF MAGNETISM. A SHORT AND CONCISE LESSON ON MAGNETISM AS IT AFFECTS THE MARINER’S COMPASS. BY CLARENCE E. LONG, MILWAUKEE. (Arranged for Masters and Pilots on the Great Lakes. ) CHAPTER IV. HE DIFFERENCE BETWEEN HARD STEEL AND SOFT IRON. A horizontal bar of soft iron at the magnetic poles has no magnetism whatever, since there it is at right angles _to the line of force. It is, in fact, in the same harmless posi- tion that the vertical bar found itself on the magnetic equa- tor. When taken, however, into low latitudes, it gradually becomes magnetic if kept pointing toward the magnetic pole, and has its greatest power in the vicinity of the equator. The red magnetism will always be found in the end which points to the north, no matter which way you may turn it, As the south magnetic pole is approached, a horizontal bar of soft iron loses force, and at the point of 90 degrees dip, has again ceased to be magnetic. In brief, vertical iron is most magnetic at the poles and horizontal iron held in the direction of the magnetic meridian, is most magnetic on the equator. Another very marked distinction between vertical and horizontal iron must here be noted. The magnetic intensity _ of the latter depends not only upon its proximity to the equator, but on the angle it makes with the magnetic mer- idian. Thus, when held in a north and south direction (correct magnetic) it is at its best; on being turned in azi- muth it loses power, and when held exactly east and west (correct magnetic) ‘has none at all. Therefore, unlike ver- tical iron, horizontal iron on board ship has a varying ac- tion upon the compass, depending on the direction of the ship’s head as well as on the position of its poles in their relation to the compass needle. This is an important distinction; but there is yet another. Horizontal iron produces the same deviation in all latitudes, for though its power varies with that ot the earth, the ratio between the two is constant; and since the first is a disturb- ing force of the needle, and the other the directing force, it follows that the deviation arising from the induced magnet- ism of horizontal iron is the same at any part of the globe. A. magnet possesses the peculiar power of producing magnetism in a bar of iron or steel without loss to itself, and so is capable of propagating its own species to any ex- tent. Therefore, when trying with a crowbar or kitchen poker, it must not be placed too close to the compass needle, as the latter, if strong, will of itself induce magnetism in the poker when, from the position in which it may be held at the time, none would otherwise exist. Thus if a com- mon spike nail be held near one pole of a powerful magnet, the latter will first induce magnetism in the nail of a con- trary name to itself, and then the daw which says that op- posite poles attract each other will come into operation, and the nail in obedience will fly to the magnet. The pro- cess of making a permanent magnet, which is. variously done by “touching” a bar of glass, hard steel with the nat- ural loadstone, with another magnet or by electricity, the one under treatment should be surcharged with the mag- netic fluid. It never, however, strength; but, after a while, settles down into a certain defi- nite state known as “the saturation point,’ which, if the steel be of the proper temper, it will maintain for years without appreciable loss, and accordingly gets named a per- manent magnet. A steel ship may be rouanetly looked upon as in itself a large permanent magnet. She became so in the process of construction; for, although the materials of which she is built are not such as by themselves retain magnetism per- manently, it is found that, when united in the form of a ship’ and subjected to percussion by riveting, etc., they ac- quire this property in a greater or less degree. After launching and reversal of the ship’s head as it was on the building slip, the magnetism undergoes very rapid diminution; but in no case does it depart entirely, and that _ which is left when the saturation point is reached is ac- cordingly styled sub-permanent. So far there is a great correspondence between the ship, taken as a whole, and the steel magnet. It is evident that the position of the poles of the ship’s sub-permanent magnetism must depend first, upon the direction of her head when building; and secondly, upon the “dip” at the part of the world in which she was built. The ship’s magnetism, properly speaking, consists of two oc aes oa -permanent and induced magnet- sm. (contaiinn IN NEXT ISSUE.) retains all its original. ANCIENT AND MODERN METHODS OF ROPE MAKING. The early methods of rope making, with brief reference to modern improvements, constitute a handsomely fin- ished book issued by the Waterbury Rope Co., of New York, from which the following description, of general interest, is taken: The word cordage is used in a comprehensive sense, to include all sizes and varieties of the article, from a har- vester twine to the largest: cable, though, strictly speak- ing, the term is hardly applicable to a rope that is less than half an inch in diameter. It is probable that rope-making was among the very earliest of human industries, since the necessity for arti- cles of the kind must have been felt by the rudest savages long before the development of the race converted nomadic tribes into semi-civilized communities. The materials first used for the purpose were probably the fibers of various grapes, the inner bark of trees, and the hides of animals cut into thongs and twisted together. Even now there may be seen in the Tyrolean Alps ropes made of twisted or braided thongs used in preference to any others, and which have descended from father to son for a hundred years. Precisely when or with whom the invention of rope-mak- ing originated it is impossible to say, but the art was cer- tainly practiced among the Egyptians more than four thou- sand years ago, as is evident from the sculptured represen- tations of the process found upon the walls of ancient structures and places of sepulture.. They made use of flax and the fibers of the date tree, as well as of rawhide, em- ployed the latter for the formation of ropes possessing suf- ficient strength to raise the huge blocks of stone which constitute the pyramids, and to place in position the colossal statues which remain to this day as monuments of rude but effective workmanship. “The rope-makers of ancient Egypt seem to have beer destitute of machinery. One man engaged a hook at the end of his twister and then walked backward away from another, who paid out the fiber of hemp, flax, papyrus, palm fiber, spartum, or whatever the material might have been. The weight enabled him to swing the twister, which was mounted on the stem he held in his hand. “In a tomb at Thebes, of the time of Thotmes III., the Pharaoh of the Exodus, is a group representing the process of twisting thongs of leather, which were fastened to the end of a tube, which revolved on a cord slung around the loins of a man who receded backwardly from the person who arranged and paid out the strands. The tube had, in all probability, a collar or sleeve which was grasped by the man, and had a bar and weight which was caused to rotate as it was swung around by the operator. “The strand of rope passed between the legs of the stool and between the feet of the man who arranged the strands and kept them from becoming tangled. “The character of the material is indicated in the man- ner which is peculiarly Egyptian, by the skin hanging up in the shop, and a man is shown cutting a continuous thong with a knife like our modern leather knife, and by the same means which we adopt, by turning the pieces of leather round as he cuts. Two coils are represented hang- ing up in the shop.” Rope-making was also well understood by other Ori- ental nations, if we may credit the historian Herodotus, who states that the Persians manufactured cables. of twen- ty-eight inches in circumference of flax and papyrus with which to aid in constructing the bridge of boats upon which the army of Xerxes crossed the Hellespont. Other writers also, mention ropes made of goat’s hair— the Tartars using hoth camel and horse hair. Among the ancient Peruvians the fibers of the maguey plant were used for rope-making, and from this material they twisted cables of sufficient strength to sustain the primitive suspension bridges which spanned ravines and rivers. Similar structures are still in common use in many tropical countries. Even the rude savages inhabiting the islands of the Pacific and Indian oceans have from time immemorial produced cordage of wonderfully excellent workmanship, considering the fact that mechanical con- trivances were almost entirely wanting, or only of the simplest and rudest description, The application of machinery to rope-making dates with- in the past century. Prior to the year 1820, hand labor, aided only by the clumsy wheels and other imperfect me- chanical contrivances pertaining to the old-fashioned rope- walk, was exclusively employed. In the year mentioned some machines were constructed in England for the pur- pose cof twisting hand-spun yarns into strands, and a few of them were imported into this country. That was con- sidered a great improvement, and as far as it went it was unquestionably a step in the right direction; but the ques- tion of introducing a radical change in the process of rope- making by the use of machinery. for spinning the threads from the raw material is due to the skill of American in ventors. The first machinery for this purpose was con- structed in Massachusetts in 1834, and since that date nu- merous other valuable improvements have originated in this country. American rope-making machinery is now ex- tensively employed in Europe, and American cordage is held in such high estimation that it is exported to all quarters of the world. Some varieties of small cordage are still manufactured by hand labor; but the substitution of machinery is be- coming so general that ere many years an old-fashioned rope-walk will be a curiosity rarelv encountered. The rope-walk was usually from 1,000 to 1,400 feet in length, and the appropriateness of its name arises from the fact that the workman was obliged to walk constantly from one end to the other. The old process of rope-making was es- sentially as follows: The fibers of the hemp were hackled or straightened out by drawing through. a’ steel-toothed comb. The workman then wound a bundle of hemp about his body, attaching one end to one of a series of hooks on a “whirl” or looper, drawing out the fibers from the bundle with one hand and compressing them with the other, ex- perience teaching the number of fibers to draw out and how to twist them so as to hold’ firmly on to the hook. He then walked slowly backward down the walk, making his yarn as he went, the spinning being done by the wheel or “whirl” turned by an assistant, the spinner seeing that the fibers were equally supplied, and joining the twisted parts at the ends. Two or more spinners might be going down the walk at the same time, and at the end two would join their yarns together, each then beginning a new yarn, and returning on the walk’to the end where the second spinner again took his yarn off the “whirl” and joined it to the end of the first spinner’s yarn, so that it continued on the reel. When a sufficient number of yarns were spun they were twisted into strands, and the strands into ropes, horse- power being usually employed. The contrast is striking between this primitive process and the methods now employed in the immense factories of the present day, equipped with steam-driven machinery of the most improved construction. / At the present day the materials employed for rope-mak- ing are various, embracing hemp, flax, cotton, manila, sisal, jute, and other vegetable fibers. Russian hemp for tarred tigging has long maintained a reputation for superiority. Its great strength and durability are attributed to the meth- od of retting the fiber under water in lieu of the mode usually adopted with American hemp, called dew-retting. Italian hemp is also of excellent quality, and for some uses unsurpassed. Manila hemp is perhaps more extensively used in the manufacture of cordage than any other mate- rial, as its great pliancy and strength particularly adapt it for the running rigging of vessels and for a multiplicity of ordinary uses. Russian and American hemp are preferred for standing rigging, owing to their ability to absorb a great amount of tar and to withstand the weather without shrink- ing or stretching. Sisal, from Yucatan, and East Indian jute are largely used for the manufacture of the cheaper grades of rope. i oor FRANK T. BULLEN’sS new book ‘‘A Sack of Shakings,”’ is published by McClure, Phillips & Co., New York. The author explains that ‘‘shakings’’ are the odds and ends of ropes and canvas accumulated during a voyage and were formerly the perquisites of the chief mate. Under this title Mr. Bullen, who is an F.R.G.S., has gathered a number of essays originally published in The Spectator where they gained considerable notice. It is upon the request of those who have read the papers with interest that Mr. Bullen now publishes them in book form. They are in- teresting notes on life at sea. Some of the essays beat these captions: The Porpoise Myth; The Floor of the Sea; Ocean Currents; Shakespeare and the Sea; Sea Etiquette; Sea Superstitions; Beneath the Surface; Voices of the Sea; The Undying Romance of the Sea; Sociable Fish, and Common Life on Board Ship.