LOCAL MAGNETIC ATTRACTION OF SHIPS. 21 



way. If these two forces are equal, then supposing the ship's 

 head to be toward the west, the needle (disregarding the va- 

 riation) would point due north-west ; if the attraction of the 

 earth exceeded the local attraction of the vessel, it would 

 point nearer north ; if the attraction of the vessel exceeded 

 that of the earth, it would point nearer west. The conse- 

 quence of this is, that the course shown on ship-board by the 

 compass differs from the actual course of the vessel, by a 

 quantity equal to the whole of this disturbing force, and which, 

 in some cases, amounts to two or three points of the compass, 

 that is to 20, 30, or 4-0, but varying with every position of 

 the ship's head, and with every change of situation upon the 

 earth, from one pole to the other. This appeared to take place 

 according to laws which bade defiance to every attempt to un- 

 ravel them. 



This local attraction of the vessel had been gradually in- 

 creasing, at first by small degrees, but afterwards in an aug- 

 mented ratio, from the period when iron was first employed 

 in considerable quantity in naval equipment ; and it was in 

 consequence of this gradual increase that the cause of error 

 remained so long unnoticed. Some obscure allusions to 

 such an effect are recorded by Dampier, Cook, and one or 

 two early French navigators. The cause was first distinctly 

 pointed out by Mr. Downie, a Master in the Royal Navy, in 

 1794?. A few years after this (the amount of iron employed 

 in vessels increasing almost every year), the lamented Captain 

 Flinders noticed the attraction more minutely : he was the 

 first to trace its connection with the dip of the needle, and to 

 point out that the effect was different in quality^ on the con- 

 trary sides of the magnetic equator, and increasing in quan- 

 tity^ as the dip in either hemisphere increased. The inquiry, 

 however, was again lost sight of, till Mr. Bain published his 

 valuable treatise on the "Variations of the Compass," in which 

 the fatal consequences attending this source of error are 

 placed in so clear a point of view, as to strike the most indif- 

 ferent and uninformed reader. Subsequently, pig-iron be- 

 came employed for ballast, the weight of which in some ves- 

 sels exceeded three hundred tons , an immense surface of iron 

 was also introduced by the admirable invention of iron-tanks, 

 to supply the place of the old water-casks. Moreover, in the 

 construction of the ships themselves, the knees, or crooked 

 parts fixing the beams to the sides ; the sleepers, or timbers 

 supporting the decks, as the joists in a house support the floors; 

 and even the riders, or interior ribs, are often all made of 

 iron. Hempen cables have also been supplanted by those of 

 iron ; and attempts were making, a few years since, to employ 



