216 



KNOWLEDGE 



[September 1, 1890. 



next, most important group of ruias esplorerl was at Beit 

 Kils. which, correstiiig a previous identification, Herr 

 Scliumacher is inchned to think may represent the ancient 

 Capitohas. The chief material of which the ruins are 

 composed in all parts is the soft friable limestone of the 

 district ; hence the remains are not in such good preserva- 

 tion as the hard basaltic structures of Bashan. At Irbid 

 (Arbela) large numbers of dolmens were discovered, 

 several of which are figured. By clearing away the soil 

 from the interior of same, masses of ashes and decayed 

 remains of bones were revealed, apparently suggesting 

 that these curious structures were used sometimes as 

 places of sepulture. A well-filled and plainly printed 

 map, and notes as to the present coaditiou, population, 

 and industries of the district are further features of 

 interest, adding to the value of what appears to be a care- 

 fully executed piece of work. 



THE MARINER'S COMPASS; ITS ERRORS 

 AND THEIR CAUSES. 



By E. Beynox. 



THE constancy of the marmer's compass in indi- 

 cating a north and south direction has long 

 furnished poets with an emblem of imdying devo- 

 tion and fixedness of affection. Modem science 

 and research, however, go far to prove that under 

 certain conditions the compass is far from being an in- 

 fallible guide. It is true that these deviations of the 

 needle are aU consequent upon the operation of the 

 natural laws of magnetic attraction and repulsion ; but in 

 our present state of knowledge the exact part played by 

 these disturbing forces is not accurately known. 



Since the adoption of iron and steel as shipbuilding 

 materials, the difliculty of guarding against local attrac- 

 tion has been vastly increased. Such importance is now 

 attached to th;se errors of the compass that it is regarded 

 as an axiom, amongst navigators, " never to regard the 

 compass as infallible, but only as a means of carrying the 

 ship safely from one observation to another.'' 



When an iron or steel ship is in process of building, a 

 definite magnetic character is impressed upon her — i.e. 

 the ship, as a whole, becomes a gigantic magnet. This 

 sub-permanent magnetism, as it is called, is at first very 

 erratic and uncertain in its action ; but twelve or fifteen 

 months' voyaging serves to rob it of its unstable character, 

 and render it constant. But this is not the only needle- 

 disturbing influence that the compass adjuster has to 

 correct. The magnetic influence of the earth will natu- 

 rally induce in a ship a magnetism that will, in north 

 latitudes, have its north-seeking pole at that part of the 

 ship nearest to the north magnetic pole of the earth. 

 From this it follows that the upper parts of vertical 

 iron, such as the upper edges of the funnel, ventilators, 

 &c., being farthest from the magnetic lines of force, will 

 be charged wth south-seeking magnetism, and will, when 

 the ship is n)rth of the magnetic equator, attract the 

 north-pointing end of the needle. At the magnetic 

 equator itself the eS'ect of vertical iron will be neutralised 

 by the equal attractive influences of the north and south 

 magnetic poles of the earth. South of the equator, how- 

 ever, vertical iron will be charged with induced magnetism 

 that will rep?l the north-seeking end of the needle. The 

 influence of this vertical iron is the great enemy to 

 thorough reliability of the compass, as its effects vary 

 with the latitude. The deviation caused by horizontal 

 iron — or quadrantal deviation, as it is termed, from its 



effects being different in different quadrants — does not 

 present the same difficulties to the corrector as does 

 vertical iron. The quadrantal error is nearly constant, 

 no matter what the latitude ; so that when once it is 

 properly corrected, that correction suffices. 



An almost perfect instrument for compensating quad- 

 rantal deviation has been invented by Chevaher Peichl, 

 of the Austro-Hungarian Navy. It consists of a number 

 of soft iron rods disposed round the compass, so that their 

 interior ends form an ellipse, of which the major axis is 

 parallel to the keel. Above this system of rods is another 

 which may be moved independently of the other. The 

 action of this corrector is due to the induction of the com- 

 pass needles on the soft iron rods. An index and scale 

 provides for the varying of this inducted power consequent 

 upon change of latitude. The sub-permanent magnetism 

 of the ship and the effect of vertical and horizontal iron 

 are pretty thoroughly compensated for when the vessel's 

 compasses are adjusted. The principle involved is merely 

 an adaptation of the old law, that the only method of 

 destroying the efl'ect of one magnetic disturbing force is to 

 introduce another magnetic force which follows the same 

 laws and has the same magnitude, but always acts in the 

 opposite direction. 



There yet remain many accidental compass-disturbing 

 influences that require the utmost watchfulness on the 

 part of na\'igators. A consensus of opinion would appear 

 to obtain aruong practical men, that a compass elevated on 

 a pole, to an altitude above the vessel's main deck equal to 

 her beam, is fi'ee from the disturbing influences of the 

 magnetism of the vessel's hull. Such a compass, when 

 corrected for the vertical iron of the funnel, &c., forms a 

 usefid check upon the standard compasses placed upon the 

 deck or bridge. Even with this safeguard, however, 

 recent research shows that the compass corrections cannot 

 be absolutely rehed on. When a ship steers upon the 

 same course for any length of time, it is found that a 

 certain magnetism is induced in the vessel as a whole. 

 This "transient-induced" magnetism, as it is called, 

 dift'ers from the sub-permanent magnetism which the 

 vessel acquired while upon the stocks or in the buUding 

 dock, in that its character is stable only so long as the 

 vessel continues upon that course. A few hours' steaming 

 in another direction serves either to annihilate or to mate- 

 rially change its nature. We have seen that the sub- 

 permanent magnetism is constant, owing, no doubt, to the 

 fact that the hammering of the plates and rivets trans- 

 formed what was originally '• transient -induced " into 

 stable magnetism. This transient force may be induced 

 in a vessel by her lying in one position in dock for a con- 

 siderable length of time. All attempts to provide an 

 adequate corrector for this disturbing influence have 

 hitherto proved abortive, and the only safeguard of the 

 na^^gator is constant observation and careful notation of 

 the results for present and future use. " Transient- 

 induced " magnetism dies away completely twenty-four 

 hours after altering the heading of the ship. Probably 

 the error of the compass thus caused is accountable for the 

 api^arently inexplicable stranding of vessels that leave 

 port when thickness of weather precludes the possibility 

 of observing landmarks, &c. It ^vill be readily seen that 

 the carrying of mineral cargoes must occasionally have a 

 great influence upon the directive powers of the needle ; 

 and it is in such cases as this that the true value of a pole 

 compass, which is practically removed from the chaos of 

 disturbmg magnetic forces that obtain on the lower part 

 of the ship, is seen. When the custom of carrying large 

 consignments of crude petroleum first came into practice, 

 many shipmasters averred that the mineral matter con- 



