354 ON THE ERRORS OF CHRONOMETERS, ARISING 



question. For, in the same proportion as the magnetism of 

 the earth, or the directive force on the compass- needle, ex- 

 ceeds the magnetism of the ship, or the deviating force, the 

 influence of terrestrial magnetism on the chronometer, must, 

 I conceive, exceed that influence exerted by the iron in the 

 ship on the chronometer. A modified action, indeed, takes 

 place where the direction of the magnetic force of the earth 

 differs from the direction of the " local attraction" of the 

 " ship ;" but yet the combined influences of the two forces, 

 however modified by direction, should, I imagine, be similar 

 on the balance of the chronometer, which vibrates in a hori- 

 zontal position, to what it is on the compass-needle, which tra- 

 verses in the same position. 



Now, the medium effect of the attraction of the iron in ves- 

 sels on the compass, in the parallels of Great Britain, does not 

 appear to exceed five degrees of deviation on each side of the 

 magnetic meridian ; it is probably a little less. The force pro- 

 ducing the deviation, therefore, is represented by the sine of 

 the angle of deviation, or 5° ; while the directive force is re- 

 presented by the sine of 85°. The relation of these two is as 

 1 to 11.35; that is, the directive influence of the earth's mag- 

 netism on the compass is 11^ times greater than the deviating 

 influence of the local attraction. Hence, the proportion of er- 

 ror due to the local attraction of the ship, would appear not to 

 exceed, in these latitudes, the eleventh or twelfth part of that 

 resulting from the earth's magnetism ; while nearer the equa- 

 tor, this proportion of error must be still less. So long as the 

 action of terrestrial magnetism, therefore, remains uncorrected, 

 it will be of little service to compensate for the error arising 

 from the local attraction. In the Polar Seasr, indeed, the force 

 of local attraction approaches the directive force much nearer 

 than in the British Seas, and in some situations very near the 



Magnetic 



