52 
Account of Captain Scoresby’s 
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 represented 
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 magnetism on the 
compass is 11 1 times greater than the deviating influence of the 
local attraction. Hence, the proportion of error due to the lo- 
cal 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 equator, this proportion of 
error must be still less. So long as the action of terrestrial mag- 
netism, therefore, remains uncorreeted, it will be of little service 
to compensate for the error arising from the local attraction. In 
the Polar Seas, 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 Poles, exceeds it ; 
but still the local attraction operates without any increase of 
force, excepting what may arise from the little augmentation of 
the magnetic intensity of the earth in those regions ; so that, in 
reality, the rate of a Chronometer in polar regions, where the 
earth’s magnetism acts nearly at right angles to the plane of the 
balance, could the effect of temperature on the instrument be 
perfectly compensated, ought to be more equable than in any 
other region, where the direction of terrestrial magnetism is more 
nearly in the plane in which the balance vibrates. 
In the important and truly scientific experiments of Mr Bar- 
low, on the effects produced on the rates of chronometers by the 
proximity of masses of iron, we have a corroboration of the pre- 
ceding opinions ; for Mr Barlow, though he observed that a va- 
riation of rate was occasioned by the influence of a mass of iron 
equivalent to the local attraction of a ship, found by no means 
so great effects as those observed by Mr Fisher. But the force 
of terrestrial magnetism acting upon a balance that is magnetic, 
is fully sufficient to account for every change of rate observed. 
Mr S. Varley, in a paper in TillocKs Philosophical Magazine, 
published in the year 1798, was the first, I believe, who showed 
