ON THE THREE REPORTS OF THE LIVERPOOL COMPASS COMMITTEE. 89 
A soft iron rod, such as that in figure 2, 
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Fig. 2. 
would give+A to the starboard compass, combined, however, with+E; and 
—A, combined with —E, to the port compass. The last arrangement is one 
sometimes found in the relative positions of the horizontal iron spindle of 
the wheel and the binnacle compasses placed near it. In compasses placed in 
the midship line of the ship, such unsymmetrical arrangements of soft iron can 
seldom have any sensible operation. In such cases A is always small; and 
when it has a sensible value, it seems more likely to arise from index error of 
the compass, or from error of observation, and may probably be best dealt 
with as such, and disregarded in the table of deviations. 
The terms B sin Z'+C cos Z! make up together what is called the “ semi- 
circular deviation.” This is the part of the deviation which it is most 
difficult to deal with, as well from each coefficient being made up of the two 
parts which we have described, which cannot be distinguished by observa- 
tions made in one latitude, as from that part of the ship’s magnetism, which 
we have treated as permanent, being in fact only subpermanent. To this we 
shali have occasion to revert in the sequel. At present we will only point 
out that +B indicates an attraction of the north point of the compass to the 
ship’s head, —B to the stern, +C an attraction of the north point to the star- 
board side, —C to the port side. 
The terms D sin 2¢'+ E cos 2Z! make up what is called the “quadrantal” 
deviation. This can only be caused by horizontal induction in soft iron. E 
can only be caused by horizontal induction in soft iron wnsymmetrically 
distributed, and is therefore, except in such cases as those represented in 
fig. 2, very small. + D may be caused by the following arrangements of sym- 
metrically arranged soft iron, in which the ship’s head is supposed to be 
directed towards the top or bottom of the page. —D may be caused by the 
same arrangements, the ship’s head being now supposed to be directed to the 
right or left of the page. 
Fig. 3 
Nt N22, N¢3. Ni4, N35. 
° —_-—-  _-———- —_—_ 09 — 
Between these various arrangements there is this most important dif- 
ference, that in No. 1 and No. 4 the directive force of the needle would be 
Increased, while in No. 2 and No. 5 it would be diminished. In No. 3 it 
