490 STAFF CAPTAIN EVANS, E.N., ON THE CHANGES OF POLAR MAGNETISM 
A93 is the mean force, or, in other words, the polar force of the ship, to head. 
X(§, is the mean or polar force to starboard, S3 2 + (5 2 the mean or polar horizontal 
force of the ship ; each in terms of the earth’s horizontal force as unit. 
is the tangent of the angle which the direction of the ship’s polar horizontal force 
makes with the line drawn to the ship’s head, or the “ starboard angle.” 
(jj is the mean or polar force downwards of earth and ship, in terms of the earth’s 
vertical force as unit ; and depends partly on the subpermanent force of the hard iron, 
partly on vertical induction. 
^2) + ^ — 1^ tan 0x1° is the heeling coefficient to windward, and represents the devi- 
ation to windward caused by an inclination of the ship of 1°, when her head is North or 
South by compass. 
The values of these coefficients, and also of a and e, the coefficients of horizontal induc- 
tion, headward and to starboard, for the several compasses are given in the General Table 
appended to this paper. 
The character of the deviations of the standard, steering and poop compasses and of 
their changes, may he thus generally described. 
In each, the S3 has originally a large negative value, caused by the ship having been 
built nearly head North, (N. 39°^ E.). This gradually diminishes as she lies in the Vic- 
toria Docks with her head to the South, but, as is usually found, shows a tendency to 
return to its original value when the ship is allowed to swing. (5, which has originally 
a large positive value caused by the starboard side having been to the South, decreases, 
and even changes its sign in Victoria Docks, but returns to its original sign and nearly 
to its original value when the ship swings at Sheerness. 
In the poop and steering compasses down to 1st January 1867, and in the Standard 
compass throughout, — except for a short period while a magnet was applied to reduce 
the deviation — there are no changes except what may be considered to be due to the 
ship’s position, and to the other circumstances adverted . to ; but as regards the poop 
and steering compasses between 1st and 26th January, the case is different ; the causes 
of the difference, and the inferences to be drawn from it, it is proposed now to 
consider. 
Early in 1866, Mr. Evan Hopkins*, C.E. applied for a patent for “An improved 
method of correcting the Deviation of Compasses in Iron Ships.” In the provisional 
specification, dated 23rd January 1866, the method is described as “ destroying the 
* Mr. Eyan Hopkins was the author of a work entitled “ On the Connexion of Geology with Terrestrial 
Magnetism,” 1844, 2nd edition, in which many singular opinions are propounded on Astronomy, Magnetism, 
and general Physics. I regret to have to speak of Mr. Hopkins in the past tense. He died in the middle of 
1867. 
