216 
MR. BARLOW ON THE ERRORS IN THE COURSE 
alluded to, iron ballast and iron tanks were perhaps scarcely known ; now we 
have, besides these, iron knees, iron cables, and above all iron capstans, besides 
various other articles of the same material, which together form such an 
attracting mass, that if we cannot allow that there was no local attraction 
forty years back, we must at least admit that it was certainly then very incon- 
siderable to what it is at present, and that navigation by compass was at that 
time comparatively on equal terms with nautical astronomy ; but since that 
period, the errors of the one have been gradually removed by improvements in 
instruments, the introduction of chronometers, and the correction of astro- 
nomical tables and data, whereas the compass still remains the same uncouth 
machine, and the disturbing forces to which it is exposed have been increased 
in, perhaps, a fourfold proportion. At all events the disturbing force is now 
considerable, and the deflection it causes in the needle, under some circum- 
stances, very great ; and as this effect is perpetually varying as the course of the 
vessel is changed, as it is also as she changes her latitude, though the course 
should remain the same, a constant attention to the amount of this error seems 
to be indispensable, at least in those circumstances where the whole safety of 
the vessel is dependent on the certainty of the courses steered, which is in fact 
always the case in a dark night, and when land is near. 
It is almost impossible to give a very popular idea of the direction and 
amount of this deflecting force. It may however be stated, that in this lati- 
tude, and in all those northern latitudes where the dip is considerable, the 
greatest deflections take place, on an east or on a west course, diminishing 
both ways to the north and south, where it vanishes ; and in all these cases the 
direction of the deflection is always to the right or left of a person, looking 
forward in the vessel, accordingly as the course of the vessel is to the right or 
left, that is to the east or west of the meridian, and it is exactly the reverse in 
a high southern latitude or with a considerable southern dip. But as we 
approach the equator, where the dip is small, the deflections at the east and 
west points vanish, and we have then four points of greatest attraction, viz. 
the N.E., N.W., S.E., and S.W., the direction of the deflection changing as we 
pass through the N., S., E., and West points ; but the deflection at its maximum 
is much less in these latitudes than in those where the dip is more considerable, 
all other things being the same. 
