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THE MARINE RECORD. 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 II. If a soft iron bar be presented endwise to either pole of a suspended magnet it will attract, though slightly, that pole; and it is indifferent which end of the iron bar is presented. In éither case the pole of the magnet “induces” a pole of an opposite character. As before stated, there is always at- traction. The power of inducing magnetism in iron diminishes as the distance increases. When the inducing magnet is slowly removed, the forces exerted by the ends of the iron bar un- dergo a corresponding diminution; and they cease to exist altogether when the magnet is wholly withdrawn, and the bar returns to its natural or neutral state. Thus the mag- netism induced in iron is temporary only. The foregoing facts enable us to account for the attrac- tion of iron by the magnet, and to reduce the phenomenon to the general principle of the attraction of opposite, and the repulsion of similar poles. When the pole of a magnet is brought near to a mass of _ iron, the immediate effect, we have seen, is to render the latter a magnet by induction, an opposite magnetism being developed in part of the mass which is nearest to the acting pole, and a similar magnetism in the part which is most remote, Now, as opposite poles attract one another, while similar poles repel, attraction will ensue between the acting pole and the nearer parts of the iron mass, and the repul- sion between the same pole and the remoter parts; and the former of these forces will preponderate, being exerted at a shorter distance. It follows from this that attraction must always result, whichever pole of the magnet be presented to the iron. Since the magnetism which has been induced in a piece of iron imparts a similar condition to a second piece in its vicinity, and that to a third, and so on, it follows that at- traction must ensue between these several pieces, although with diminished energy, as they are more distant from the acting pole; and the first will support a second, the second a third, and so on, if only the weights of the successive pieces be proportionate to the diminished force. A mag- netic chain can thus be formed, having two poles. The fol- lowing experiment illustrates the matter: Let a few pieces of iron rod, or a few soft iron nails be taken. If one of these be placed in contact with the pole of a permanent steel magnet, it is attracted to it and becomes a temporary magnet; it now, in turn, supports a second piece; this, in ttirn, a third piece, and so on, until a chain of four or five pieces is built up. Each of these pieces of iron is itself a magnet. If it be the north pole of the magnet to which the pieces are attached, the end of the first piece in contact with the steel magnet will have south, and the other end north, magnetism, and this last will develop in the nearest end of the next piece of iron, south magnetism, and so on. But if the steel magnet be removed from the top of the chain, all fhe rest drop off and are found to be no longer magnetic. A piece of soft iron laid with one end on the end of a magnet practically becomes part of the magnet, simply extending its length, and its further end being of the same polarity as that of the magnet to which it is joined, : POLARITY. The influence that gives directive force to the compass needle; causing it to point to the magnetic north, is called polarity. It is customary to regard the polarity which exists in the north end of the compass needle, red polarity, whence it follows that south, blue polarity, must pervade the re- gions of the terrestrial north, because it is a law of nature that bodies similarly magnetized will repel, while those dis- similarly magnetized will attract each other. sade SOFT IRON. As regards magnetism, is iron which becomes in- stantly magnetized to its full capacity, when exposed to the influence of any magnetized body, and which loses its magnetism instantly when the influencing body is removed. Now, the earth may be regarded as a huge magnet having the same properties as the natural and arti- ficial magnet and possessing the two poles (north and south polarities), and a neutral line, or magnetic equator. _ The north magnetic pole of the earth’s magnetism was sit- uated, at the time of discovery, in the northwestern extrem- point; they can start from anywhere. ity of Hudson Bay, and has been actually reached by man, it being discovered by Sir James Ross, of the English Navy, on June 1, 1881; whilst the other lies to the south of Tas- mania, but owing to impenetrable barriers of ice, has not quite been reached, but so nearly that its position was with- out difficulty ascertained by calculation. The magnetic poles of the earth are not diametrically opposite each other; the north magnetic pole being situated 1200 nautical miles south of the true or geographic pole of the earth, and: the south magnetic pole 900 nautical miles north of the true south pole. Like other magnetic bodies, the two terminal portions of the earth’s magnetism are its poles, where the attraction is greatest. Unlike the true, or geographical poles, which are represented by a mere point, the magnetic poles include a considerable area of the earth’s surface, amounting per- haps to 50 square miles. They revolve very slowly around the true poles of the earth, but for what reason no one knows, it being due to some unknown law. Tue TRUE or GEOGRAPHICAL Pores are the extremities of the earth’s axis, the two points of 90 degrees north and south from the equator; or where all the meridians of longitude cross or cut one another, or the points where there is no longitude at all or 360 degrees of it, just as you please. These are also the points where there is only one direction; being situated on the true north pole of the earth, you could only look or go in one direction, namely, south; therefore, the direction south must commence at the true north pole of the earth. The conditions would be identically the same to an observer situated on the true south pole of the earth, except the one direction there being north; therefore, the direction north must commence at the true south pole of the earth. An observer situated at either of these two points during the summer season of each, would always have the sun on the meridian, bearing either due north or due south, but if he should take one step from the pole, the above conditions would instantly change, for he would then have all the directions of the compass, and the sun would change its bearing readings, the same as other positions on the earth’s’ surface. The reader of this will probably wonder where the directions east and west commence. These directions, as well as all others, with the exceptions of north and south, have no commencing MAGNETIC EQUATOR OF THE EARTH. | Now, as the earth has two magnetic poles, it must also have a magnetic equator. This magnetic equator is a sinu- ous curve encircling the earth, and crosses the geographical equator in two places nearly diametrically opposite to each other, something after the fashion of the ecliptic. The mag- netic equator crosses the earth’s equator on the eastern side of the Atlantic, at about the meridian of 11 degrees West, and the other in the Pacific, about longitude 168 degrees West. Its greatest divergence from the true, or earth’s equator, is in Brazil. Now, the earth’s magnetic equator, acts the same as the equator does of any other magnet. It separates the two polarities of the earth’s magnetism from each other. It is a neutral line where there is no vertical force issuing forth. ‘This means that at any place on this line on the earth’s surface the dipping needle (to be ex- plained later) rests in a horizontal position. THE DIRECTION OF THE FORCE OF TERRESTRIAL MAGNETISM. This is estimated in two co-ordinate planes, the one hori- zontal, the other vertical; the former of which the geo- graphical meridian is taken as the initial line. It is meas- ured by the direction of the magnetic needle suspended to move in each of these planes. The needle hung so as to move in the horizontal plane is called the “Declination” or “Variation Needle’; that hung so as to move in the verti- cal plane the “Inclination” or ‘Dipping Needle.” To recapitulate: The needle of the mariner’s compass moves in the horizontal plane, and receives “its directive force from the magnetic poles of the earth. The dipping needle moves in the vertical plane, and this is controlled by the earth’s line of force, or the earth’s vertical magnetic force, which varies at different places upon the earth’s sur- face. The earth has no vertical force on the magnetic equator, and at all places on this line the dipping needle takes a true horizontal position ; or in other words the dip is zero. At the magnetic poles of the earth the dipping needle points vertically downwards, or in other words, the dip is 90 degrees. If the dipping needle is carried toward the equator from either magnetic pole of the earth it will gradually approach a horizontal position until the magnetic equator is reached, when a true horizontal position is at- tained. APRIL, 18, I90l. SS LONG TOWAGE SERVICES. If there is any section of the world where towage service has been brought to a stage nearly bordering on perfection. it isin the locality of the Great Lakes. One of the most modern appliances and a piece of mechanism which has led in no small measure to this stage of efficiency is towing by means of an automatic steam towing machine adapted to use _ flexible steel wire tow lines and controlled so as to give and, take according to the weight and surging brought on to the tow line even by a string of 8,000-ton’steel tow barges such as are now being generally built and used in the Lake Su- perior iron ore trade. ; While claiming full credit for the splendid efficieney of the lake towing arrangements, it is perhaps just as well to note what is being done in this line of work elsewhere and, we take great pleasure in quoting from the Liverpool Jour- nal of Commerce, of recent date, the work of some of the tugs owned at that port, as follows: ‘The arrival at Swansea of the Liverpool Tug Co.’s Blazer with the French steel barque Ville de Dijon in tow has al- ready been reported in our shipping news. This ship was on her maiden voyage, and one of the large type of four- masted sailing ships, her register tonnage being over 2,000. She was built at Havre last year, and was at the time of her disaster on her homeward voyage from Mueo, New Caledonia for Swansea, with a cargo of nickel ore. Besides losing her spars she developed a leak, and considerable time had been lost since September 16 last, when she was towed into Monte Video by the German steamer Amasis. She had to be part discharged and re-loaded, and otherwise underwent tem- porary repairs, besides which legal proceedings have caused some difficulty at Monte Video over the salvage claim. The famous tug Blazer was commissioned to tow the Ville de Dijon home, her previous record marking her out for the undertaking, for a tow of over 6,200 miles is not a very fre- quent occurrenee, and calls for capabilities possessed by very few tugboats afloat. The tug left Liverpool on 2nd January, and arrived at St. Vincent, Cape Verde Islands, on Ioth Jan- uary, and after taking in coal proceeded, arriving at Monte Video on 28th January. Owing to the Ville de Dijon not not having completed arrangements for a crew, the start was postponed until the 4th February. The tug and her tow then began the long journey via Pernambuco, distant some. 2,100 miles from Monte Video, which port was touched at for coaling purposes. St. Vincent, Cape Verdes, was reached Maxch 9, and more coal was taken in there, the voy- age being resumed the following day. Between this port and Madeira some heavy weather was experienced but noth- ing occurred to mar the success of the tow, Madeira being: being called at asa final coaling station. The last stage of the trip, viz., Madeira to Swansea, being successfully accom- plished, the Blazer thus breaking her own record. This tug has frequently figured in great towing feats, her excellent work during her career being of the most creditable charac- ter. Some of the most important jobs on which she has been engaged are probably the towing of the large steamer Mo- zambique from Ceara to Rio, as far back as October, 1893; the steamer Cambrian, in conjunction with another tug, from St. Michael’s to Liverpool, in 1897; the steamer Augus- tine, St. Vincent, Cape Verdes, to Liverpool, late the same year, which voyage, covering a distance of 2,345 miles, was made in 9 days 7 hours; whilst more recent work includes: Cabenda (s), West Coast of Africa to Middlesbrough; Den- ton Grange (s), Las Palmas to Tyne; Hornby Grange (s),- Vigo to Tyne; Burgundia (s), Fayal to Marseilles; Tremont (s), St. Michael’s to London; Kilmaho (s), Ferrol to Glas- gow, Lokoja (s), Horta, Azores, to Havre; Duca di Galliera (s), St. Vincent to Genoa; Kinfauns, Azores to Cardiff and Gergovia (s), St. Vincent to Marseilles. For time and dis- tance, however, ihe latest performance is the Blazer’s record ane that record has never been paralleled by any other craft. The Cruizer, the largest and latest addition to the Liver- pool Steamtug Co.’s fleet, has just completed the towage of the steamer Indra, 6,057 tons, from Aden to Liverpool, about 4,700 miles (towing the vessel through the Suez canal by her- self without assistance), which, we believe, is the first time that any one tug has been allowed to attempt this feat. We know that these tugs are fitted with the highest grade of flexible steel wire tow-ropes, but whether they have the patent automatic steam towing machines or not we can’t say, if not, we should advise the Liverpool Tug Co. to write the American Ship Windlass Co. of Providence, R. I., manufac- turers of this appliance for easy and safe towing. ee oO oe Some of the experts on lake navigation declare that owing to the effect of the flowing of water through the drainage canal at Chicago, the Straits of Mackinac will be cleared earlier than usual, many expecting that the advantage gained will amount to ten or fifteen days. This fact is said to ac- count for the early opening of the season by some of the northern lines. It is argued that the flow of water through the canal increases the speed of the flow through the Straits and thus clears the ice out more quickly. The usual time for the opening of the Straits is April 20.