m THE lEON SHIPS OF THE EOYAL NAVY. 
339 
of this formula ; obsendng, that in those investigations the coefficients A and E are 
eliminated by the assumption that the soft iron is symmetrically distributed on each 
side of the fore-and-aft line of the ship. 
The meaning of the several coefficients is as follows : — 
A is a constant deviation, real or apparent ; . -j- if easterly deviation is in excess, 
— if westerly; when real, it arises from the induction of the horizontal force of the 
earth on masses of soft iron unsymmetrically distributed ; when only apparent, as in 
nearly every case recorded, it is considered due to prism, or index error of the compass 
on board, or of the compass employed on shore in obtaining reciprocal bearings, as also 
local disturbance of the latter ; and, when the de\dation has been observed by the bear- 
ing of a distant object, that the latter has been imperfectly determined. 
B and C are due to the combined effects of the permanent magnetism of the hard 
iron ; the deviation produced by which varies inversely as the horizontal force at the 
place, and that induced by the vertical part of the earth’s force on the soft iron in the 
ship, the deviation produced by which varies as the tangent of the dip. This deviation 
has been termed “ polar-magnet ” by the Astronomer Eoyal, and “ semicircular ” by 
Mr. A. Smith, from the nature and appearance of the curve when graphically delineated. 
B represents that part of the combined attraction acting in a fore-and-aft direction ; 
if before the compass, — if abaft it ; that is, the north end of the needle is attracted 
to the bow in the former case, and to the stern in the latter. 
C is that portion of the combined attraction acting in an athwart-ship direction; 
+ if the north end of the needle is djawn to the starboard side, — if drawn to the port 
side of the ship. 
D and E, known as the permanent coefficients, from their (theoretically considered) 
unchanging value in all magnetic latitudes, are due to the horizontal induction of the 
soft iron in the ship, and produce a “ quadrantal ” deviation ; the term multiplied by 
D having its four maxima at the intercardinal points, and that by E at the cardinal 
points. 
The values of D are considered by theory to arise from masses acting in a fore-and-aft, 
or transverse direction ; + from those before or abaft, — from those on either side. 
E is in general so small as to be practically unimportant. 
Tables of Magnetic Elements of various Ships in Her Majesty's Navy, Iron and Wood- 
built. 
The results of the analysis of the Tables of Compass Deviation are thus arranged ; — 
Table I. The iron-built vessels arranged according to tonnage : this analysis includes 
every Deviation Table of such vessels recorded in the Admiralty. 
Table II. includes the floating batteries ; one iron-built, and the others wood-built 
plated with iron. 
Table III. A selection from the wood-built screw steam-vessels, embracing those of 
large, medium, and small dimensions. 
2z 2 
