THE COMPASS-NEEDLE ON THE DEVIATIONS OP THE COMPASS. 
177 
The quadrantal de\aation of the ship being known, the compasses should be fixed at 
that distance apart at which they produce the deviation required to be corrected. 
The semicircular deviation is then to be corrected in the usual way, except that the 
semicircular deviations of both compasses may be corrected by the same two magnets, 
placed respectively fore-and-aft and athw^art-ship, and half-way between the two com- 
passes. 
The adjustment of the compasses may be made with less preliminary observation, but 
with more calculation, in the following way : — 
For each compass, let the amount of deflection which it will produce on a test-needle 
at a distance of, say, 1 foot 6 inches when directed towards the centre of the test-needle 
and at right-angles to it, be ascertained and recorded. 
A pair of such needles will produce on each other at a distance of 1 foot 6 inches a 
quadrantal deviation of three-fourths of the recorded amount, and at any other distance 
a de\’iation which bears to three-fourths of the recorded deviation the proportion which 
the cube of 18 bears to the cube of the number of inches required. 
In conclusion, we would urge most strongly the importance of selecting for the 
position of the compass or compasses a place where the deviation is moderate in 
amount, and nearly uniform for some distance around the compasses. No compass 
should be placed in a position in which the original deviation in England much exceeds 
two points. 
2 c 
MDCCCLXI. 
