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8 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 VI. \ TO MARK THE POLES OR ENDS OF A MAGNET. To tell which end is the north pole and which end is the south pole of a magnet. If you present one end (it makes no difference which end) of a bar magnet to the north point of the compass needle, an attraction takes place, then that end of the magnet must be of an opposite name to that end of the compass needle attracted, or a south or blue pole. If repulsion took place under the same conditions, that is, -if the north end of the compass needle was repelled, or driven away from the magnet, the two poles must be like in name, ora north or red pole. Just try this at the first opportunity. This is the manner in which magnets are marked and paint- ed. Now, to prove that you have marked the magnet cor- rectly, just fasten a piece of cord to the magnet’s center of gravity, or nearly so, and hang it up on shore in a place free from local attraction, and when it ceases vibrating it will come to rest in a north and south direction. The end you marked north should point in the direction of north and the end marked south, toward the south. Try it. You can likewise prove the principles of the dipping needle in the same manner. To make a dipping needle for experi- mental purposes: Take a fine piece of steel, say 2 1-2 inches long by 1-4 of an inch wide and 1-64 of an inch thick. Have, both ends drawn to a point, as nearly equal as possible, then bore a small hole, large enough for a thread, through its center of gravity, or as near to it as possible, then hang the needle up by the thread. If you have the exact center of gravity the needle will remain in equilibrium (at a balance) in any position in which it may be placed. If the needle does not balance, file off from the heavy end until it does: After accomplishing this, then have the needle tempered and magnetized. If it then be suspended and be free to assume any direction, it will no longer be indifferent, but will place itself in a particular vertical plane called the magnetic mer- idian, and will take a particular direction in that plane. Whatever this angle makes with the horizontal plane will be the amount of dip at the place of observation, and a line through the poles at that time is called the line of force at that place. Before suspending the needle by the thread after magnetization, mark its ends according to the foregoing rule. After suspension the north end will be found to dip toward the earth. HOW TO MAKE MAGNETS. Take a good quality of bar steel, say 3-4 of an inch wide by 3-16 of an inch thick and 8 or 10 inches in length. Have the extremities for about two inches tempered glass hard, and then magnetize it by placing it on top of a dynamo and leaving it there ten or fifteen minutes. Place it so that about half of its length projects over the edge of the dy- namo. The stronger the current the stronger the magnet. Magnetizing them with an electrical coil is by far the best, but the above method will answer for all ordinary purposes. ODDS AND ENDS. Compasses poorly placed are of course more affected by the iron in the ship than those well placed. In some iron and steel vessels the compass is affected so as to make it practically useless on some headings. Sometimes large and perplexing deviations are caused by the magnetism induced in vertical soft iron near the com- pass, and if so it must be corrected by vertical wrought iron bars. A table of deviations is as necessary after adjustment as before, and the error of the compass should be frequently found and the table corrected. It has been sometimes supposed that fogs and peculiar states of the atmosphere cause magnetic disturbance and af- fect the compass needle. There is, however, no ground for this supposition. Electric storms are liable to disturb. the compass needle, and lightning has the occasional effect of rendering the needle useless; the aurora borealis also af-. fects the compass needle, but there is no authority that its effect is felt on the lakes. Attempts have been made to intercept or screen the action of the ship’s magnetism in causing deviation of the compass, No effective magnetic screen has yet been produced which is not destructive of the usefulness of the compass for the pur- THE MARINE RECORD. pose of navigation, and it may be stated with confidence that there never will be. If they did succeed in insulating the needle from the magnetism of the ship, we should by the same means intercept the magnetism of the earth, and thus the compass would be rendered absolutely useless. Even if such a medium did exist it would be of no value as a screen. A FEW IMPORTANT CAUTIONS. Keep all iron and steel as far from your compasses as possible. Bear in mind that magnetic influence will not be stopped by placing anything between the compass and the iron or steel. It will pass through a stone wall. See that the magnets once placed by the adjuster, are let se- verely alone. They should never be touched, unless you know, of course, what you are changing them for. Make it a cast iron rule to ascertain the deviation of the compass at every opportunity. Remember, that however well a compass may be adjusted, observations for deviation should be made just the same, as the magnetism in the boat is liable to slight changes at any time, which can only be ascertained by comparisons with azimuths. TO TELL A GOOD COMPASS, A simple test for a compass is to place it in position, not- ing the reading thereof, and with a pocket-knife, or piece of steel, deflect the needle, remove the attraction quickly, and note the time it takes to come to rest at the exact point it started from. If the card is very slow in coming to rest, or does not return to the exact point it started from, some- thing is wrong with the compass. Either the magnets are weak, the weight too great on the point of support, causing friction, or the jewel cap may be defective. In purchasing a compass, always give it this test. Remember, the compass that will show the quickest number of vibrations in a given number of seconds is the best instrument for any and all purposes. VARIATION OF THE COMPASS. “Variation of the Compass,” is the term used exclusively to denote the correction of the compass (not compass error) due to the earth’s magnetism, arising from the fact that the magnetic north and south poles of the earth do not coincide with the true or geographical poles. Variation may be re- garded as the angle measured at the place of the observer, between the earth’s nearest true and magnetic poles. A compass needle, perfectly free from the effects of iron or other magnetic substance, in obedience to the earth’s influ- ence will rest in the plane of the magnetic meridian. It is also defined as the angle between the north and south line of the card and the correct magnetic north and south line. Variation, unlike deviation, affects every point of the com- pass the same amount when in any one position upon the earth’s surface. In compass compensation the variation, of course, is never adjusted, as it is external to the vessel, and of different amount and name at different places. Variation is always to be found on the chart, and it is always the same at the same place, with the exception of a small annual in- crease or decrease, usually from 3 to 5 seconds. DEVIATION OF THE COMPASS. “Deviation of the Compass,’’ is the term used exclusively to denote the correction of the compass, due to the attrac- tion of the iron of the ship, in her equipment, boiler, engine, funnels, etc., or in her cargo. It is entirely independent of the variation, being produced by objects within the ship. LOCAL ATTRACTIONS. “Tocal Attraction,” is the term used to express the dis- turbance of the compass by magnetic influence existing out- side of the ship, such as iron cranes, water pipes or pillars, steel and iron work, ore dock, steel elevators, etc., etc., as may be found in docks and’ other confined water spaces; also the magnetic attraction of one iron vessel on the com- pass of another, should be termed “local attraction.” It will be understood that, as its name implies, the disturbing quantity is purely local, and that as the vessel gets outside and away this disturbance has no effect whatever on the compass. Thus we have the ‘three terms “Variation of the Compass,” ‘‘Deviation of the Compass,’ and “Local Attrac- tion,” in a nutshell. WRONG EXPRESSIONS. Some persons use the terms variation and deviation as meaning one and the same thing, but the well-informed sail- or makes a great distinction between the two for the reason that variation is caused wholly by the earth’s magnetism, and the deviation by the iron in the ship. Deviation is often mixed up with local attraction, and vice versa, the two ex- pressions being used indifferently to mean the same thing. This is wrong as they are entirely distinct. The ‘first named is due to causes within the ship herself; the latter to out- May 16, 1901 ee —— side influences. Variation, deviation and local attraction, are as distinct from one another as black is from white, and each arises from an entirely different cause. They should never be mixed up with one another in this manner. (To be continued.) A NEW BRITISH BUILT STEAMER. A few particulars regarding a British built steamer just — completed will be found of interest to many of our readers. ‘i The principal hull dimensions are—length, 452 feet; breadth, " 54 feet; and depth, 35 feet 3 inches. The vessel has been — specially designed as a general cargo boat and to the require-_ ments for a passenger certificate. The Peleus, of the Holt | line, is specially adapted for carrying the bulky packages of the China trade, the main hatch pillars having been dis- pensed with and box girders substituted, with the hatch coamings forming part of same. The system of construc- tion is carried out ina modified degree in the remaining portions of the vessel. The stern-post and rudder area special feature, the dead-wood at the foot of the stern-post ‘ being completely cut away, thus allowing a free run of water to the propeller. A wrought steel tube forms the stern-post, from which the rudder is hung, and the arch of the remain- ing portion of the stern frame consists of a steel plate of a “U” section. The rudder is of the balanced type, and is entirely constructed of plates and angles, excepting the upper portion of the stock, which isa forging. The deck erections consist of poop, topgallant forecastle anda long bridge, on which are placed deck-houses for the accommo- dation of captain, officers and engineers, and the crew are berthed under the forecastle. The deck equipment includes 17 steam winches and 25 derricks, one being capable of lift- ing 35 tons. The engines are a powerful set of the triple- expansion type, with the most modern improvements in auxiliaries. Steam is supplied to the engines at a pressure of 190 pounds from two large double-ended boilers, fitted with a system of forced draught, having closed stokeholds. The trials on the measured mile proved very satisfactory, an average of 14 knots being attained. oO | rt EASTERN FREIGHT REPORT. Messrs. Funch, Edye & Co., New York, report the condition of the eastern freight market as follows: Our freight market continues in the same unsatisfactory condition and there are no new features toreport. Grain chartering has been restricted owing to the fluctuation in the price of cereals together with the scarcity of corn at the sea- board, which has prevented shippers from effecting new business. The majority of the steamers recently seeking business on this coast have finally been diverted to the Gulf for timber or to the British Provinces for deals, and in some instances an advance on previous rates has been secured. General cargo business from the Gulf continues practically ata standstill, whilst from the Atlantic cotton ports the re- quirements are also very limited. The export of coal to the Mediterranean continues neglected for the time being, but a fixture has just been made to Rio Janeiro and there are fur- ther ir quiries of a similar nature. : In regard to sail tonnage we have positively nothing new of interest to report, and can only repeat that the market continues firm, but with very little activity. Rates remain unchanged. : _— OOOO Ol SS CASUALTIES. | The crew of the schooner Narragansett, abandoned in Lake Huron, reached Harbor Beach in safety. The Narra- gansett was bound from Detour to Port Huron for repairs and had no cargo, In the brisk winds Sunday the vessel waterlogged and the crew were forced to take to the boats when twelve miles off Point Aux Barques. That was three o’clock Monday morning, and they were in the wooden yawl boat for over twenty-four hours. It is thought the schooner sank not long after being abandoned. She was an old schooner, having been built in 1861. Her last owner was John Leisch, of Milwaukee. Capt. Kean, of the steamer Advance, reports coming across the schooner Narragansett, of Milwaukee, in the middle of Lake Huron, twenty-five miles due east of Point Aux Barques. The crew had abandoned her and she was full of water. He put three of his men on board and towed her towards Goderich for about three hours, when she sud- denly foundered eleven miles northwest of Goderich. The wooden passenger steamer Bon Voyage, Duluth, on fire and beached near Houghton, Mich., Lake Superior, Friday, May 10. One family, being all the women passen- gers, were drowned.