54 
MR. AIRY ON THE OBSERVED DEVIATIONS OF THE COMPASS 
where BAD is the true azimuth of the polar-magnet. In other cases it is convenient 
to refer the azimuth of the polar-magnet to the disturbed direction of the compass- 
needle or BE, and then the polar-magnet-deviation is given by this formula: 
sinACB=^.sinEBF, 
CA 
where EBF is the apparent azimuth of the polar-magnet. In either case, the law 
which connects the polar-magnet-deviation with the azimuth (true or apparent) of 
the polar-magnet is what I shall call “the law of polar-magnet-deviation.” 
Secondly: the disturbing effect of the polar-magnet, whose power is represented 
by AB, may be completely neutralized by attaching to the same frame (whether it 
be a ship, or an experimental wood frame, &c.) which carries that polar-magnet, 
another polar-magnet in the opposite position, its power and direction being repre- 
sented by the line kb. 
Thirdly : if, instead of the polar magnetism of a steel bar, the disturbing force 
upon the compass be that of the transient induced magnetism in a nearly spherical 
mass of soft iron possessing no permanent magnetism, placed in the same horizontal 
plane as the compass ; and if NOS, fig. 2, represent 
the position taken by the needle under the action of 
terrestrial magnetism only; then if the mass of soft 
iron be in either of the positions Mj, Mg, M3, M4, it 
will not disturb the needle NOS: if the mass of soft 
iron be placed either in the quadrant between Mi and 
M2 (as at Mg) or in the quadrant between M3 and M4 
(as at Mg), it will make the point N deviate towards ; 
and if the soft iron be placed in eithei" of the remaining 
quadrants (as at M; or Mg) it will make the point N 
deviate towards n^. The amount of deviation is pro- 
portional to the sine of double the angle of azimuth of the disturbing mass, that is to 
the sine of double the angle MjOMg, or MjCMg, &c. If the disturbing mass be 
carried round the circle in the direction M1M2M3M4M1, the deviation of the needle 
(estimated positive when the point N is moved towards the right, and negative 
when towards the left) will in the four quadrants have the signs -1 -j . The 
deviation following this law 1 shall call “quadrantal deviation.” 
Fourthly: the deviation produced by the mass of soft iron at M5 will not be 
corrected by placing a similar mass at Mg (which, instead of correcting the deviation, 
will double it), but it wiU be corrected by placing a similar mass either at or at mg, 
the angles MgCmg and MjOmg being supposed to be right angles. Similarly, the 
deviation produced by the mass at Mg will be neutralized by a similar mass either at 
ni^ or at mg, if the angles MgOA^s, MgOmg are right angles. Thus the “ quadrantal 
deviation” may be corrected by attaching to the same frame (whether it be a ship, 
or a wooden experimental frame, &c.), which carries the mass that produces the 
“ quadrantal deviation,” another mass, at the same level as the compass hut in an 
Fig. 2. 
S / 

M.3 
