Barlow's Essay o» Magnetic At traction. 133 



tudes may frequently be steering a point 'out of iier Rupi)oscd 

 course ; and it requires but little knowledge of navigation to be 

 well convinced of the immense importance of such an error, 

 particularly in the Channel, and in cloudv weather, wlien the 

 compass is the seaman's only friend and guide. 



It appears moreover from the memoir ot Captain Sabiur, and 

 the same was also pointed out by Captain Flinders and Mr. Bain, 

 that the error from local attraction is not only different with the 

 ship's head on difft-rent points of the compass, Init that it varies 

 also in different latitudes; the quantity depending upon the in- 

 clination of the dipping needle at the place of observation. Cap- 

 tain Flinders indeed does not appear to have made a correct 

 estimate of the effect of the dip in changing the ([uantity of the 

 deviation ; but he had the honour of being the first to establish 

 the connexion between them, and thus conferred a lasting obli- 

 gation upon the nautical profession, and has given to his name 

 a permanent and perspicuous situation in the annals of that 

 science. An actual deviation caused bv the action of the guns 

 being thus rendered obvious by the concurrini: testimony of so 

 many distinct observers; and tlie only rule ever yet proposed for 

 the correction having been distinctly shown to be inadequate for 

 the purjjose, by all latter oi)^crvations ; it became a fair and lau- 

 dable subject of contest with men of science, to discover some 

 formula, or other method, for correcting these anomalies in all 

 parts of the world. 



Mostly, hov^cver, the attempts were confined to the forma- 

 tion of certain enqiirical formuke, from the results of observa- 

 tions on ship-l)oard; but these, it is obvious, are at present too 

 limited for such a purpose, when we consider that every degree 

 of latitude gives rise to a new series of deviations. 



Mr. Barlow pursued a different course. Being placed in a si-' 

 tuation where there is perhaps a greater accumulation of manu- 

 factured iron than in any place in the world, he proposed to 

 avail himself of this ojiportunity, and to undertake a regular 

 scries of experiments on dilferent masses of iron, fixing his com- 

 pass at different distances, and in every variety of position ; and 

 Ijy this means he has succeeded, not onlv in discovcriu!;- an ex- 

 tremely simple method of corrcctinii the deviation in (juostion, 

 but also in developing several magnetic laws which i-.re higldy 

 curious and interesting. It is indeed surprising, considering the 

 interest which the science of magnetism has for more than a cc.i- 

 tury excited, that no course of experiments of this kind lias been 

 before nndertaUcn, and tlt-at instead of cxamiiinj;; in all cases the 

 action of magnet on magnet, the intjniry had not suggested it- 

 self, of determining tlic laws between j/hun iron and the com- 

 I 3 pass. 



