MAGNETISM. 



Mr. Downie, Master of H. M. S. Glory, 

 in which there is the following passage : 



I am convinced that the quantity 



and vicinity of iron in most ships have 

 an effect in attracting the needle ; for 

 it is found by experience that the needle 

 will not always point, in the same direc- 

 tion when placed in different parts of 

 the ship. Also, it is rarely found that 

 two ships steering in the same course, 

 by their respective compasses, will go 

 exactly parallel to each other ; yet these 

 compasses, when compared on board 

 the same ship, will agree exactly*/ 



(246.) The next observations on this 

 subject were those of Captain Flinders f, 

 who, whilst surveying the south coast 

 of New Holland, in H. M. S. Investi- 

 gator, in 1801 and 1802, remarked con- 

 siderable differences in the direction of 

 the magnetic needle, when there was no 

 other apparent cause for them, than the 

 differences in the direction of the ship's 

 head. This occasioned much perplexity 

 in laying down the bearings, as it was 

 very difficult to find the proper allow- 

 ances to be made for this deviation of 

 the compass in estimating them. With 

 a view of trying how far an alteration 

 in the disposition of the iron might 

 tend to remedy this source of error, 

 Captain Flinders first removed two guns, 

 which had stood near the compass, 

 into the hold, and afterwards fixed the 

 surveying compass exactly a-midships 

 upon the binnacle ; for at first it was oc- 

 casionally shifted to the weather-side as 

 the ships went about; but neither of 

 these two arrangements produced any 

 material effect in preventing the devia- 

 tions of the compass. When the ship's 

 head was to the east, the deviation was 

 westward ; and the contrary, when the 

 ship's head was to the west : when it 

 was nearly north or south, no deviation 

 was perceptible. These differences, aris- 

 ing from a change in the direction of 

 the ship with regard to the points of the 

 compass, were less considerable as he 

 proceeded to lower latitudes ; and on ap- 

 proaching the line of no variation, upon 

 the south coast of New Holland, the 

 deviations of the compass were smaller 

 than either before or afterwards. In 

 reasoning on the cause of these devia- 

 tions, he supposes * the attractive power 

 of the different bodies in the ship, which 

 are capable of effecting the compass, 



* Walker's Treatise on Magnetism, published in 

 1794 ; quoted by Mr. Barlow in his Essay on Mag- 

 netic Attractions. 



i Philosophical Transactions for 1805, p. 186, 



to be collected into something like a 

 focal point, or centre of gravity ; and 

 that this point is nearly in the centre of 

 the ship, where the shot are deposited, 

 for here the greatest quantity of iron is 

 collected together/ He further sup- 

 poses that this point is endowed with 

 the same kind of attraction as the pole 

 of the hemisphere where the ship is ; 

 consequently, in New Holland, the south 

 end of the needle would be attracted by 

 it, .and the north end repelled. On this 

 hypothesis, which appears to be the true 

 one, he explains the phenomena he had 

 observed, and also deduces from it as 

 a necessary consequence, that the devi- 

 ations of the compass, arising from the 

 attraction of the iron in the ship, must, 

 when the ship is on the north side of the 

 magnetic equator, be directly the reverse 

 of those he had observed in the southern 

 hemisphere; that is, the north end of 

 the needle would be attracted, and the 

 south end repelled. This theory was 

 confirmed by other observations, made in 

 the same ship, in the British Channel. 



(247.) The observations of Captain 

 Flinders excited considerable attention 

 at the time they were published ; and a 

 course of experiments was, in conse- 

 quence, made, by order of the Admiralty, 

 in various ships in the Nore. It was 

 found that, in every ship a compass 

 would vary considerably in its position 

 on being removed from one part of the 

 ship to another. Although the general 

 fact was completely established by these 

 experiments, they did not then lead to 

 any further investigation, until the sub- 

 ject was again brought into notice by 

 Mr. Bain, who, in a useful treatise which 

 he published on the Variation of the 

 Compass, placed in a striking point of 

 view the fatal consequences which might 

 attend this source of error. The atten- 

 tion of the public was also particularly 

 drawn to the subject at this time, in con- 

 sequence of the proposed expeditions to 

 the Arctic regions, from which it was ex- 

 pected that much important information 

 would result with regard to terrestrial 

 magnetism. The local attraction of the 

 vessels sent out on these expeditions was 

 made a particular object of inquiry ; and 

 the results of the numerous experiments 

 made for that purpose, are detailed by 

 Captains Ross and Parry in their ac- 

 counts of their respective voyages ; and 

 also by Captain Sabine, in a paper in 

 the Philosophical Transactions *. It is 



* For 1819, p. 112, 



