210 



(JNDULATORY FORCES. MAGNETISM. [EFT or roo OH TUB COMPASS. 



America ; and a vast fund of valuable information hat 

 bean acquired. At the present timo the observations are 

 registered by mean* of photography ;* and thui tlio in- 

 oossaut oare required on the part of observers, atid the 

 chance of erroneous results, aro entirely avoided. Tin-re 

 U also the great advantage of no intermission being 

 required, at, by means of properly prepared paper, a 

 lamp with a reflector, and a clockwork arrangement, tliu 

 indication* print themselves, and the photograph, on 

 removal from the instrument, shows precisely all tho 

 variations which have token place, and the exact uiomunt 

 of the occurrence. One of the best instruments of this 

 kind is that which was constructed by Air. Brooke, for 



' ireenwich Observatory. 



Returning to tho subject of the compasses of a ship, 

 wo may notice some special influences to which they are 

 subject, and the remedies which have been employed to 

 obviate such. They may bo divided into accidental, local, 

 and, possibly, atmospheric. 



It would bo impossible to enumerate the occidental 

 circumstances which may arise, to cause the derange- 

 ment of a ship's compasses. Articles of iron placed near 

 them aro a frequent cause; and an intelligent captain 

 once informed us, that, for four hours, he had been 

 steering, on a dark night, out of his course, owing to 

 the stupidity of a cabin-boy, who hod placed a box of 

 knivet close to the compass. 



The local influences to which the noodle is liable on 

 board ship are of great importance, and depend on the 

 nature of the material of which the ship is built, the 

 cargo, tho engines of a steam-vessel, and tho position of 

 the two latter in reference to the compass. As soon 

 as the ship is completely loaded, she is token to any 

 convenient place in which she con turn round in an 

 entire circle being held by her anchor as the tide rises 

 and falls. In this process the direction of the needle is 

 carefully watched in reference to any distant aud fixed 

 object, and the variation, if any, is noted. Her com- 

 passes are then adjusted that is, magnets are placed 

 near, so as to entirely counteract all the influences which 

 may affect them on board, and to wlu'ch we have just 

 alluded. If the cargo be undisturbed until the vessel 

 reach her final destination, and no accidental or at- 

 mospheric inlluenco be exerted, such adjustment 

 would be a sufficient safeguard for the whole voyage. 

 If, however, a portion of tho cargo be discharged at 

 intermediate ports, it may happen that the adjustment 

 previously effected will become a source of danger ; 

 because the disturbing element being removed, its iu- 

 tend'sd remedy ueces -.irily becomes a source of disturbance 

 itself. This is a matter of common occurrence on board 

 coasting vessels : we observed an instance of this, some 

 years ago, in a steam-vessel which hod left port with a 

 large cargo of iron on board. This was discharged at an 

 intermediate port ; and tho needle was thereby affected 

 to the extent of between two and three degrees. It is 

 highly probable that many of the shipwrecks which 

 annually occur round our coast, may owe their origin to 

 this constantly-occurring circumstance. 



With respect to atmospheric influences, especially as 

 regards fog, (great diversity of opinion exists. Many 

 persons insist that the noodle ceases to act in a dense 

 fog ; whilst others equally deny its influence : and thus 

 when any vessel is lost during foggy weather, the evi- 

 dence offered as to tho cause, is always accompanied 

 with the special views of the witnesses on each side. 

 In the absence of any reliable data, we shall mention two 

 instances of our own observation, taken within a fm 

 days, on tho east and west coasts of England and Scot- 

 land. 

 lit a voyage from London to Edinburgh, in on iron 



8ce aiilr, p. 1C3. 



1 screw steampr of about 000 tons burthen, and with a 

 general cargo, a douse fog set in immediately after 

 leaving the Pool. This continued for twenty-eight 

 hours ; during wlu'ch, for safety's sake, the speed was 

 never allowed to exceed |fivo miles per hoar, and the 

 anchor was cast frequently, owing to the positive danger 

 of proceeding at all There were three compasses on 

 board one at tho stern, another on the hurricane dock, 

 and one in tho cabin. The writer compared their indi- 

 cations, repeatedly, each hour during the time mentioned ; 

 and in no case did either, or all of the compasses, vary 

 from each other to the extent of one degree. A pocket 

 compass, the needle of which was delicately hung, agreed 

 with the average of the ship's compasses. There was 

 not the slightest motion of the vessel, except from the 

 vibration of tho screw. So far, no indication of the 

 action of fog was afforded. In another instance, 

 however, which occurred in a voyage between Ayr ami 

 Compbelton, at the month of the Clyde, very dill, 

 results were noticed. The voyage was commenced in a 

 dense fog, and no special cause of disturbance was on 

 board, the vessel being built of wood, and no iron was con- 

 tained as cargo. After steering direct westward tov. 

 the Isle of Ai ran for about on hour, with the intention 

 of calling at a port at its southern end, it was discm 

 that the vessel had been steered gradually towards the 

 north, although the compass had indicated a westerly 

 course. The needle, in fact, varied about 90 from its 

 proper indication; and this increased until the set i in- 

 sun appeared in the north-east, instead of slightly south 

 of west. During tho whole time the coihposs hail 

 shaken so as to cause the needle to move freely ; Unities, 

 the vibration from the paddles ought also to hove been 

 quite sufficient for that purpose. At the end of about 

 two hours' time from the commencement of tho voyage, 

 a brisk wind sprang up, and almost immediately after- 

 wards the compass-needle veered round to its proper 

 direction, and gave no more cause for anxiety. ID 

 the former, and in this instance, the circumstances were 

 exactly tho same ; for a breeze sprung up and cleared 

 away the fog at the end of the twenty-eight hoxirs, 

 yet the compasses showed no change of direction from 

 that which they had indicated previous to the clearing- 

 up of the weather. Tho observations in each case were 

 conducted with great care, and with that scrupulous 

 hesitancy as to theoretical opinion, which the study of 

 experimental science makes a habit; and wo could not 

 find, on free conversation with the captain of eaeh vessel, 

 that any extraordinary circumstance of any kind WAS 

 present which could vitiate the result. 



We may, however, state, that three or four iron vessels 

 have been lost between the Isle of Arran and the 

 side of the coast of Argyllshire, near ('anipbi-ltoii, since 

 the above observations were made. In each case the 

 cause was assigned to tho fog acting on tho compasses. 

 Wo may venture to suggest that there may be some 

 local cause in the constituents of tho rocks, <fce., of 

 that neighbourhood, especially as we learned from an 

 intelligent scientific observer, that one or two m.c- 

 rock near Campbeltou, have a decided effect on tho 

 magnetic needle. However, wo merely throw out the 

 idea as a suggestion rather than as expressing any 

 opinion on the matter, which, of course, can onlv be 

 decided by minute aud scrupulously exact observations. 

 The efl'ect of electrical currents wo have already iv- 

 ivd to; and we shall have to enlarge considerably on 

 tho subject in tho next chapter. They aro scared 

 much moment during tho voyage of a vossol, beeanhe, 

 Ixjing local with respect to extraordinary oivnn. 

 uid the position of tho vessel continually varying, their 

 force and intensity can bo practically, except iu very 

 special cases, of little consequence. 



