494 STAFF CAPTAIN EVANS, E.N., ON THE CHANGES OF POLAE MAGNETISM 
The action of the several forces may perhaps be more clearly apprehended when they 
are represented graphically as in Plate XXXI. In this, fig. 1 represents the projection 
of lines indicating the amount and direction of the magnetic forces which act on the three 
compasses on the horizontal plane. Fig. 2 represents the projection of the same lines 
on the fore and aft vertical plane The figures are drawn to a scale in which one-fourth 
of an inch represents one foot, and also one-tenth of the earth’s horizontal force. The 
lightly dotted parts represent iron. 
P, Q, E represent the positions of the port, the poop, and the starboard steering com- 
passes respectively ; P 'p, Qq, Hr represent the projection of lines representing the mag- 
netic forces of the ship at these positions on the 1st January 1867 ; P 'p' , Q(f, EP the 
same projection on the 26th January 1867 ; P 'p", Qq ", Er" the projection of the addi- 
tional forces introduced in the interval. 
In fig. 1 the near approach to parallelism and equality in Yp, Q^, Er indicate a 
distinct cause of magnetic force. The lines Pp", Qq", Er" produced backwards nearly 
meet in a point about 5 feet abaft the poop compass ; indicating that the additional 
force is introduced at or near that point. A similar convergence of the line Qq", Er" in 
fig. 2 indicates that the point lies in or near the poop deck, and suggests that it arose 
from the magnetization of the central part of the iron beams of the poop deck, modified 
possibly by some magnetization of the beams of, or stanchions supporting the upper deck. 
On the 28th February, and preparatory to the ship being navigated to Devonport, the 
deviations and magnetic forces were observed. The deviation of the Standard compass 
being too great for the safe navigation of the ship, and the deviation of the starboard 
steering compass being so great as to make it practically useless, it was necessary to 
reduce their semicircular deviations by the application of fixed magnets. The process 
employed, and which is that generally employed in the Eoyal Navy, is identical with 
one of the two methods described by the Astronomer Eoyal in his well-known paper on 
the magnetism of iron-built ships (Philosophical Transactions, 1839, see page 196), and 
may be described as follows. 
The coefficients 93, X being found by observation, or where necessary S3 and (5 
being found by observation, and X being estimated ; we have A^/SS 2 -j- (E 2 the tangent of 
the semicircular deviation when the polar force acts to the east or west of the compass, and 
^ the tangent of the “ starboard angle.” If we desire to correct the semicircular deviation 
completely, a magnet of suitable size, adequate power, and proved permanence is 
selected from those in store at the Compass Observatory at Woolwich. The distance 
above or below the card at which this magnet, when placed East and West, will produce a 
deviation of which the tangent is X\/ S3 2 + ($ 2 , is ascertained by actual trial. The magnet 
is then inserted into the pedestal of the Standard compass at the ascertained distance 
immediately below the centre of the card and in the direction of the starboard angle, 
the poles being so placed as to counteract the polar magnetism of the ship. 
As in newly built ships the polar force is generally undergoing a process of gradual 
diminution, it is generally considered best not to correct entirely the semicircular devia- 
