68 



MAGNETISM. 



should be necessary to use a compass 

 in any other part of the ship, a refer- 

 ence should be made to the standard of 

 comparison, and the difference, if any, 

 in its pointing noted and allowed for ; 

 a certain degree of uniformity being 

 found to obtain in the effects of the 

 local attraction on a compass thus con- 

 fined to one spot, enabling a navigator 

 to form a sufficiently correct judgment 

 of the different amounts of variation to 

 be allowed with it on each change in the 

 direction of the ship's head *. 



(266.) Not only are the compasses on 

 ship-board disturbed by the magnetic 

 attraction excited by the iron existing in 

 the vessel ; the chronometers also are 

 affected by the same influence. The 

 sudden alteration 'in the rates of chro- 

 nometers at sea had been frequently 

 noticed by intelligent seamen, but had 

 been generally ascribed to the motion 

 of the vessels. The true cause was first 

 pointed out by Mr. George Fisher, who 

 accompanied Captain Buchan in his 

 voyage to the Arctic Regions, in the 

 year 1818, and who gave an account of 

 his observations on this subject to the 

 Royal Society t. He found that the 

 chronometers on board the Dorothea 

 and Trent had a different rate of going 

 from what they had on shore, even 

 when these vessels had been frozen in, 

 and therefore when their motion could 

 not have contributed to that variation. 

 It appeared that this effect could be 

 attributed only to the magnetic action 

 exerted by the iron in the ships upon the 

 inner rim of the balance of the chro- 

 nometers, which is made of steel. A 

 similar influence was perceptible on 

 placing magnets in the neighbourhood 

 of the chronometers. This conclusion 

 was confirmed by the experiments made 

 for this purpose by Mr. Barlow, who 

 ascertained that masses of iron, devoid 

 of all permanent magnetism, occasioned 

 an alteration in the rates of chronome- 

 ters, placed in different positions in 

 their vicinity. The alterations varied 

 according to the positions of each chro- 

 nometer with relation to the magnetic 

 equator of the masses of iron to whose 

 influence it was subjected, and was al- 

 ways uniform in the same position. In 

 the case of the chronometers on board 

 the Dorothea and Trent, their rate was 

 always accelerated. Mr. Barlow found, 

 however, that this depends on the cir- 



* Parry's Journal of a Voyage for the Discovery 

 of a North- West Passage, &c., Appendix, p. cxviii. 

 f Philosophical Transactions for 1820, p. 196. 



cumstances of the case, for in other in- 

 stances they were retarded. He sug- 

 gests that great care ought to be taken 

 to keep the chronometers on board of 

 any ship out of the immediate vicinity 

 of any considerable mass or surface of 

 iron. They ought not, for instance, to 

 be kept in the cabins of the gun-room 

 officers, which are on the sides of the 

 vessel ; as probably a strong iron knee, 

 or even a gun, will be found at a very 

 inconsiderable distance from the spot 

 where the watch is deposited. 



Mr. Barlow proposes to ' rectify this 

 error by a method similar to that which 

 he employs for the correction in the 

 compass, namely, by previously ascer- 

 taining what the effect of the ship's iron 

 is upon the rate of the chronometer. 

 This may be done : by means of a box 

 or pedestal, on the top of which is a 

 convenient receptacle for the chrono- 

 meter, and in the side of which a brass 

 pin is fixed, to carry the compensating 

 double iron plate, employed to repre- 

 sent the action of the ship's iron on the 

 compass. Then, having ascertained the 

 rate of the chronometer in the usual 

 manner, let the rate be again taken 

 while it is placed on the pedestal. The 

 plate should generally be kept at the 

 distance of about a foot from the ver- 

 tical line, through the centre of the dial ; 

 and its centre should be about the same 

 depth below the plane of its balance. 

 The rate thus obtained will be a very 

 close approximation to the ship's rate 

 of the instrument, provided care be 

 taken to keep it out of the immediate 

 action of any partial mass of iron, and 

 to place it in the same direction with 

 respect to the ship's head as it had 

 with respect to the iron plate when its 

 rate was determined *. 



3. Of the Azimuth Compass. 



(267.) The purposes to which the 

 azimuth compass is applied, and the 

 general principles of its construction, 

 have already been stated in 240 ; but, 

 for the sake of those who are desirous 

 of making practical use of it, it will be 

 necessary to enter into a fuller detail. 



The ordinary azimuth compass is 

 represented in fig. 63. The semicircle 

 AB is fixed by a screw at its middle, or 

 lowest point, to a stand at the bottom 

 of the outer box, containing the whole 

 apparatus, in such a manner as to ad- 



* Barlow's Essay on Magnetic Attractions, p. 



