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MAY 9g, Igor. THE LAW OF MAGNETISM. A SHORT AND CONCISE LESSON ON MAGNETISM AS IT AFFECTS THE MARINER’S COMPASS. BY CLARENCE E. LONG, MILWAUKER. (Arranged for Masters and Pilots on the Great Lakes. ) CHAPTER V. THE DIFFERENT KINDS OF DEVIATION, THEIR CAUSES, AND HO\ TO COMPENSATE EACH, Semi-circular deviation, so-called because both of its phases occur in opposite semi-circles; for instance, if the deviation is easterly on north it will be westerly on south, and vice versa; and if easterly on west it will be westerly on east, and vice versa. Semi-circular deviation is caused by the sub-permanent magnetism in the ship and by the in- . duced magnetism in vertical iron. The Permanent portion of the ship's magnetism which causes Semi-circular or Po- lar Deviation, is compensated by steel magnets, whose mag- netism is likewise permanent; and that portion due to in- duction in vertical iron, which goes and comes with change of latitude, and likewise causes semi-circular deviation, is compensated by vertical bars of ordinary wrought iron, which similarly become magnetic by. terrestrial induction, and are influenced in a corresponding degree by such _ changes of latitude as both may be exposed to. THE MARINE RECORD The principles are identically. the same as in the case of the athwartship magnet. QuapranTaL Deviation, so called, because all of its phases occur in the four quadrants of the compass, namely, NE, SE., SW., and NW. Quadrantal deviation is caused by the transient or inductive magnetism of horizontal soft iron, such as iron deckbeams, the iron spindle of a wheel, etc. It is nothing with the ship’s head north, south, east and west, and greatest on the quadrantal points, It is gen- erally easterly in the NE. and SW. quadrants, and westerly in the NW. and SE. quadrants of the compass. Quadrantal deviation remains unchanged in all magnetic latitudes, and provided that the iron in the ship be of good quality, the quadrantal deviation will be little if at all altered by lapse of time. TO CORRECT THE QUADRANTAL DEVIATION, The semi-circular deviation having been adjusted as near ‘as possible, the quadrantal deviation is compensated by soft iron generally in one or other of three forms, namely; Hori- zontal cylinders, like clock weights, globes, like the round shot of Nelson’s time, or masses of small, close-linked iron jack chain. This last is a chain enclosed in brass boxes and placed generally athwartships of the compass. When the quadrantal correctors are placed athwartships, in either of the three forms above named, they correct for easterly deviation in the NE, and SW. quadrants and west- UNITED STATES MONTHLY SHIPBUILDING RETURNS. TREASURY DEPARTMENT, OFFICE OF THE COMMISSIONER OF NAVIGATION. The Bureau of Navigation reports 128 vessels of 63,159 gross tons were built in the United States and officially num- bered during the month of April, 1901, as follows: WOOD. STEEL. TOTAL. SAIL, STEAM SAIL. STEAM No Gross No Grors, No, Gross. No Gross. No. Gross. Atlantic and Gulf................ 23 5,472 30 POOH aces Gleaner ws 5 13,998 68 21,165 RACING asi ie se vs Siasras he scales 10 3,077 21 TOGS P| oinaieler pees I 23 32 4,198 Great alesis. Cece vse cose lee hes Cee ue tus 3 1,570 2 4,558 9 30,324 14 36,452 Wester Rivers... cic. a 4 70 10 TA il tans aiarayeil sowrobcwe ticks na aeivaneed| ise aise tickers 14 1,344 Wotale ah suis... hoes oe pee 47 8,619 64 5,637 cael 4,558 15 44,345 || 128 | 63,159 The largest steel steam vessels included in the figures are: NAME. GROSS TONS. WHERE BUILT. OWNER. WNOVORIAN hen cies is Sane Se ees, 5,597 Chester, Pa. American-Hawaiian S.S. Co. Wietitien eo in sas es Ce ee ree Siar Corgan wares 3,719 Lorain, Ohio. American Ship Building Co. URIRC Tre ce uen te vas clays seach eee RIOR 3,719 oe an " es a us Williams Browsing > aiid se teak Gaekis coos. vacua § 4,998 Chicago, Il]. Federal Steam Ship Co. IPAH COMPS WATHER so ccc ec cb Sek ay cise Sols ccc ao. 3,062 West Superior, Wis. Superior Ship Building Co. Wore Ne As seats cae is criti Geen Ste wis o's <issace ne a 3,062 us i ss be ss se cs MUBTS etc ste Cena come tee ee cence eee « 3,748 Wyandotte, Mich. Detroit: ....‘‘ sf ss Mavide MuewWhitney, 300s ee oad Be, ees 4,626 si es David C. Whitney. Hsperanza ......... PE Catrina dail asd Neo tohin Sunk 4,702 Philadelphia, Pa. ‘N.Y. & Cuba M. S. Co. The foregoing figures do not include craft without motive power of their own. From other sources than construction one vessel of 622 gross tons was added to the merchant fleet, as follows: RIG. NAME. Hindoo 622 "WHY OFFICIALLY NUMBERED. Foreign Wreck. GROSS TONS. The bar magnet placed athwartship, with its center in the fore-and-aft line and exactly at right angles to the com- pass needle, adjusts for the deviation on north and on south. If the deviation had been westerly on north it would be easterly on south, and the magnet so placed as to correct the deviation on north would also correct for the opposite deviation on south and for the same amount when the vessel comes to head south. The present system of compass com- pensation presupposes the attraction causing deviation to be equal on north and south, but in a majority of cases the at- traction is unequal, due to more iron placed on one side of the compass than on the other. Supposing the case of a vessel having, say, one point of easterly deviation on north and on south only one-half a point westerly deviation. Now, it is plainly to be seen that if we adjust for the one point of deviation on north it corrects also for one point on south, but as there was only one-half a point on south in the first place, by adjusting for one point introduces an error of one- half a point on south, while north is correct. If we adjust- ed for the half point on south, then this would leave a half a point on north. The only thing to do in a case of this kind is to halve the error, that is, split it, leaving as much on one point as on the other point, The bar magnet placed fore-and-aft with its center in the athwartship line corrects for the deviation on east and west. erly deviation in the NW. and SE. quadrants. Now, it is the earth’s magnetism that makes magnets out of these soft iron correctors by induction, and it all depends in which position they are relative to the magnet meridian. When the vessel heads either north or south correct magnetic these correctors are non-magnetic, for the reason that they are at right angles to the magnetic meridian or earth’s horizon- tal force and therefore powerless to affect the compass. When the vessel heads east or west correct magnetic the soft iron correctors, being in line with the magnetic merid- ian, are rendered strong magnets,-but they cannot affect the compass as their poles are in line, or parallel, with the needles of the compass; but instead, they give the compass needles just that much more directive force. : Now, reason out how these correctors effect their pur- pose and which end is north polarity and which end is south polarity. With the ship’s head NE., the port ends have red or north magnetism. Why? Because those ends are point- ing towards north, and being soft iron become readily mag- netized, but opposite in name from the magnetism of the north magnetic pole of the earth, which has south polarity. The starboard ends then must have blue polarity. The end of the corrector on the port-hand side next to the compass has blue polarity and attracts the north end of the compass needle. The end of the corrector on the starboard side next to the compass has red polarity and attracts the south end of the compass needle. In this position they both correct for easterly deviation, one assisting the other. SW. these correctors would have the same effect; but on heading the vessel NW., their starboard ends require red magnetism and compensate westerly deviation. They would also compensate westerly deviation when heading SE. These correctors are too frequently absent, and it should be remembered that they very essentially improve the action of the compass, not only diminishing the deviation but in- creasing the directive force. As already stated, these cor- rectors, heing composed of soft iron, readily become mag- netized by induction from the earth’s force. So long as the ship is upright, these globes are inoperative on north and south courses (where the Heeling Error amounts to the most), but when she heels over the lower part of the cor- rector on the weather side of the ship is raised approxi- mately to the level of the compass needles, and having red polarity helps to neutralize the blue polarity of the side of the ship. In this it is assisted by the blue polarity of the upper half of the lee corrector. HEELING ERROR DUE TO MAGNETIC FORCE BELOW THE COMPASS, Heeling changes the position of the iron in the ship and what was before horizontal now inclines towards ‘the ver- tical position. In north magnetic latitude, the upper or weather ends of athwartship iron beams draw the north end of the needle to windward. The position of the vertical iron is also changed, producing a corresponding change of devia- tion. Heeling error, as a rule, throws a vessel to windward on northerly courses and to leeward on southerly courses. The heeling error is greatest on northerly and southerly courses, and the least on east and west courses. ADVANTAGE OF HEELING ERROR COMPENSATION. It is.very essential that the Heeling Error be compen- sated. To show the advantage a vessel has whose heeling error is adjusted over another where it is not so, just sup- pose them to be on northerly or southerly courses in a rough beam sea. In the one case, each time the ship rolls, the vertical magnetic force below the compass will come out now on one side of the needle, and now on another, caus- ing the card to be alternately pulled to starboard and port at every roll; and should this pull happen to coincide with the period of vibration due to the motion of the ship, the swing of the card will be so great as to render it perfectly useless. In such cases a man ignorant of the science of com- pass adjustment will be almost certain to attribute the ex- cessive swing to some inherent fault of the compass, and inwardly curse the maker. On the other hand, the properly adjusted compass will remain comparatively steady under all circumstances, and, any little swing will be due to purely mechanical causes. It has already been shown that the Quadrantal Correctors help to correct the whole of it; and likewise, another small constituent of the Heeling Error is corrected by the Flinder’s-bar. From the foregoing, the ship-master will see how necessary it is that the Semicir- cular magnetism, causing the deviation on east and west, should be resolved into its constituent parts and each com- pensated by the means suitable to it. Enough has been said to convince even the most skeptical that it is absolutely necessary to understand the laws of magnetism before compass adjustment can be understood, let alone undertaken. It is not necessary that a shipmaster, whose experience is limited to the new vessels he commands and whose duty to his owners calls for. as many trips us possible during the season of navigation, should be able to adjust his own compass, but he should understand the laws of magnetism and the theory of the science so that when he employs an adjuster he knows whether the adjuster is doing the work according to the laws upon which it is based. A shipmaster should also be familiar with the Azi- muth work so that he can ascertain the deviation of his compass at any time of the day or night. No matter how correctly a compass may be adjusted, it will only remain so for a limited time; it requires only a short interval of time, vibrations, shocks, or strains to change the magnetic force in the vessel so as to make the compass point incorrectly. (To be continu: d.) ee TOLEDO FUELING DOCKS. Stenley B. Smith & Co. of Detroit apparently intend in- vading the Toledo field and for this purpose have secured two fuel docks on the Maumee river, one at the Ohio Cen- tral Railroad Co.’s dock on the east side, and the other on the Hocking Valley dock on the west side of the river. Heading,