September 1, 1890.] 



KNOWLEDGE. 



217 



tained in the oil was a prolific source of compass error. 

 Subsequent experience, however, has neither disproved 

 nor substantiated this opinion. Now that the petroleum 

 trade has assumed such enormous developments, this is a 

 phase of the subject which requires investigation, and can 

 only be solved by mariners recording in detail the conduct 

 of the compasses when placed in close juxtaposition with 

 such a cargo. The influence of land masses, when vessels are 

 engaged in cruising in close proximity to the coast, is un- 

 doubtedly a bondfiile source of compass deviation, the import- 

 ance of which cannot be over estimated. Many rocks contain 

 large masses of iron in one form or another which must 

 exercise an attractive or repellent influence upon the com- 

 pass needle brought near them. The results of many re- 

 corded observations show that magnetic rocks in the 

 northern hemisphere attract the north-seeking end of the 

 needle, while south of the magnetic equator the contrary 

 effect is produced. Eed sandstones, as containing ferru- 

 ginous matter in the form of a colouring oxide, produce 

 considerable deflection upon the needle brought near 

 them. But it may be ai-gued that it is very rarely that a 

 ship would approach so near a shore as to come within 

 the zone of magnetic disturbance. This is true. But at 

 the same time the ship, while sufficiently distant from the 

 land to preserve her compasses intact ft'om deflection 

 caused by horizontal attraction or repulsion, may yet be 

 separated vertically, by but a few fathoms, from rocks of 

 the same nature. Perhaps the most striking recorded 

 example of deflection caused by rocks in the sea-bed over 

 wliich a vessel was sailing is that of H.M.S. surveying 

 vessel Meda. This ship, while engaged in surveying on 

 the north coast of Australia near Cossack, reported that 

 when three miles off the shore and in eight fathoms of 

 water her compass was steadily deflected 30° for a 

 period of fifteen minutes, during which time she sailed over 

 one mile. The variation marked down for the east coast 

 of Madagascar is 11° W. to 12° W. But the French men- 

 of-war, which are frequently cruising in these waters, find 

 that the variation near the shore at St. Mary's Isle is only 

 go Qj. r^o ■^_ rjij^g attraction of the north-seeking end of 

 the compass by the magnetic rocks at the bottom is held 

 accountable for the 5° of alteration. Similar results have 

 been recorded of places upon the coasts of New Zealand." 

 To many sea-going people this will no doubt be a revela- 

 tion, for it is only very recently that the influence of rocks 

 under the bottom of a ship has been credited with the 

 power of causing needle delation. What the exact rela- 

 tionship is between vertical magnetic attraction and the 

 hdiizontal deflection consequent upon it, is at present 

 undetermined. But further researches in the matter will 

 ^bo watched with the keenest interest by the nautical 

 profession as well as the scientific public. 



Another alleged cause of compass deviation is fog. A 

 large percentage of the strandings which occur during fogs 

 are ascribed to unknown errors in the compass. A careful 

 perusal of these cases, however, establishes the important 

 fact that no authentic case of fog disturbance has ever 

 been adduced except to account for an apparent mistake in 

 reckoning. Observations taken to test this tlieory show 

 there is little or no foundation for it. The most sensitive 

 instruments at Kew show no alteration in their directive 



* There is a similar area of magnetic deflection in the Adriatic 

 which was surveyed at the reijuest of Father Secchi. It seems pro- 

 bable that such local disturbances are due to the action of hot 

 springs or volcanic vents, which alToi't rocks containing magnetic 

 oxide of iron or other magnetic matter and cause them to become 

 magnetic radially with respect to the source of heat. In other 

 situations magnetic strata take up the induced magnetism of the 

 earth, and would rot sensibly disturb the direction of the compass. — 

 A. C. U. 



power during the densest fogs. Staff-Commander Creak, 

 superintendent of compasses at the Admiralty, expresses 

 the opinion that bad steering in all probability accounts 

 for the compass de\iation attributed to fog. When a ship 

 is in sight of land, the objects nsible on shore form points 

 of reference by which any looseness of steering can be 

 compensated. Should a fog come on, and the same care- 

 less hand remain at the wheel, it is possible that now, when 

 the correcting obiects on shore are no longer visible, a 

 falling ofl' of the vessel from her course may result in her 

 stranding. It is just possible, also, that in the case of some 

 of the strandings atti-ibuted to compass error caused by 

 fog, the real cause of the de^•iation may be magnetic rocks 

 lying in close proximity to the vessel. 



Some of the causes of compass errors would be amusing 

 were they not productive of the most disastrous con- 

 sequences. Some little time ago a vessel was wrecked in 

 the Channel during thick weather. It transpired, at the 

 inquiry into her loss, that when the master made the 

 ob-servation by which the fatal course was fixed that sent 

 the vessel ashore, the man at the wheel was wearing a 

 magnetic belt. It was consequently assumed that the 

 compass needle was deflected by " .Jack's belt," and that 

 when he left the wheel the deflection disappeared, and 

 the compass, hadng assumed its normal condition, led the 

 ship on to destruction. The metallic ribs of an umbrella 

 thoughtlessly placed near the standard compass by a 

 passenger have been known to produce a deviation of the 

 needle, and consequent erratic steering. 



The two accidental causes of error alluded to above 

 point their own moral. Officers and crew, or even pas- 

 sengers, may carry about their persons some hidden source 

 of magnetic force which, if taken near to the steering 

 compass, will undoubtedly influence it, possibly without 

 the -error being detected for the time being. It is e\-ident 

 that ships' compasses will have to be jealously guarded 

 from any possible deflection from these hidden influences. 



Now that electricity is coming into such general use on 

 board steamships, too much care cannot be taken to pro- 

 cure for the compass absolute immimity from its influence. 

 With a perfect insulation there is no danger ; but many 

 cases confirm the oiDiuion that where the installation is 

 carried out on unscientific principles, or where the in- 

 sulation is imperfect, the compasses may be rendered 

 absolutely dangerous. 



Speaking of this phase of the subject. Captain Creak 

 has laid it down as an axiom that the dynamos employed 

 shotfld be placed at least S.i feet from the compasses. On 

 board the Kitiilinites the compass, though 10 feet away, 

 was afi'ected by the polarity of an Edison machine. Both 

 poles of the machine should be at an equal distance from 

 the compass, w-hile the lead and return wires should be 

 placed as near together as possible and carefully insulated. 

 A most interesting example of what may occur with im- 

 proper installation is evidenced by experiment on H.M.S. 

 lioyalist. One of the wires from a dynamo was made 

 to pass under the standard compass at a distance of some 

 13 feet. The return wire was separated from it by some 

 5 feet. The passage of the electric current along the wire 

 caused a deflection of some 8°, which increased with the 

 strengthening of the current. The compass in this way 

 was transformed into a galvanometer, registering any 

 variation in the strength of the current with the utmost 

 exactness. \'essels whoso electric lighting system provides 

 but a single wire lead and utilizes the hull as return, very 

 materially increase the risk they rim of having their com- 

 passes thrown into error. 



In conclusion, it must not be lost sight of that each ship 

 possesses an individual magnetic character of her own, and 



