THE COMPASS-NEEDLE ON THE DEVIATIONS OP THE COMPASS. 
167 
Before leaving this part of the subject, it is proper to say, with reference to the mode 
of correcting the compass proposed by Mr. Aiet, that the peculiar errors considered in 
this paper are of a kind and amount which could hardly have been contemplated by 
Mr. Aiet. The sextantal error becomes wholly insensible, even with a large semicir- 
cular deviation, when the magnets being in the same horizontal plane, are more than 
six lengths of the needle from the compass, or when being above or below the compass 
they are considerably within that distance. The octantal errors become insensible when 
the soft iron correctors are more than two lengths of the needle from the compass. It 
is not probable that Mr. Aiet contemplated that the correctors would be brought 
within these distances. It is only with the large semicircular and quadrantal deviations 
of iron ships, which the compass adjusters of the present day are not afraid to correct 
by magnets and soft iron brought in close proximity to the compass, and with the 
needles of extraordinary length, which are considered suitable to ships of extraordinary 
size, that the errors in question become sensible or material. 
New Mode of Correcting the Quadrantal Deviation. 
The correction of the quadrantal deviation by Mr. Aiet’s method, although theoreti- 
cally more perfect than the correction of the semicircular deviation, is practically more 
embarrassing, When made, it remains perfect, notwithstanding any change in the inde- 
pendent magnetism, or in the geographical position of the ship; but the increased 
use of iron in the construction even of iron vessels, and perhaps some change in the 
quality of the iron used, has greatly increased the amount of the quadrantal deviation. 
From 1° and 1° 6', the quadrantal deviations of the Eainbow and Ironsides on which 
Mr, Aiet’s observations were made in 1839, it has increased to an average of 3° or 4° in 
iron Ships of War, and of 7°or 8° in some iron vessels of the Mercantile Marine. The 
correction of such deviations by soft iron requires, from the comparative weakness of 
induced magnetism, the employment of large masses of iron, brought so near the com- 
pass that large octantal errors are caused in the single-needle compass, and not wholly 
avoided by the use of the Admiralty Standard compass, and opening further sources of 
error in the independent magnetism of the corrector and the magnetism induced in it 
by the compass-needle. 
A mode of correcting the quadrantal deviation by a permanent magnet, which shall 
furnish the requisite amount of force without being brought into too close proximity 
to the compass, is therefore a desideratum. Such a correction cannot be obtained from 
a magnet in a fixed position in the ship, which can only correct deviations proportional to 
the sines and cosines of odd multiples of the azimuth of the ship’s head ; but it occurred 
to Mr. Evaxs that it may be obtained from the reciprocal action of two compasses 
arranged as in an ordinary double binnacle. It is easily shown mathematically that two 
such compasses of equal strength produce on each other a negative quadrantal deviation, 
together with (in the case of single-needle compasses) a small octantal deviation, with- 
out introducing any other error ; and as the quadrantal error to be corrected is, in all 
