IN WOOD-BUILT AND IRON-BUILT SHIPS. 
79 
by a quantity which at station No. 25 would produce a maximum effect of 2°. The 
change in the voyage between No. 24 and No. 25 is nearly insensible. 
The Vulcan. 
26. H+NX 9-63= -0-603 8=4-0-046 
27. H — Nx6-19=— 1-236 8=4-0059 
In this instance, as in some others, we feel greatly the want of observations after 
the ship’s return, to inform us whether the ship’s subpermanent magnetism has really 
undergone a change. Assuming that it has not (and it is certain that 8 has not 
sensibly changed), then H has a sensible negative value and N a sensible positive 
value: and the correction for all stations would be effected by magnets and masses 
of iron as has been described for the Mseander. But it would probably be better to 
rely solely on adjustible magnets for the correction of the polar-magnet-disturbance. 
The Simoom. 
28. H4-Nx 9-63= 4- 1*364 S=-0-451 
29. H-Nx6-19 = 4-l-055 S=— 0-184 
The change in the value of 8 would produce at the station No. 29 a maximum 
error of about 3° 40'. If the whole change in the compound headward force depended 
upon H, that change would also produce an error of nearly the same magnitude ; and 
the combination of the two would produce, as the total result of the change of sub- 
permanent magnetism, an error of about 5° 30'. But it is probable that N has some 
positive value, and that the change of H is not so great. 
I shall now state what appear to be the just practical inferences from the preceding 
investigations. 
1. At any place, the deviation of the compass in any ship, whether wood-built or 
iron-built, may be accurately represented as the effect of the combination of two 
forces, of which one alone would produce a disturbance following the law of polar- 
magnet-deviation, and the other alone vmuld produce a disturbance following the law 
of quadrantal deviation. 
2. Consequently, at any place the deviation of the compass may be accurately 
corrected by well-known mechanical methods ; namely by a magnet in the athwart- 
ship direetion, fixed at a distance determined by trial, for correcting the deviation 
when the ship’s head is N. or 8.; by a magnet in the head-and-stern direction, also 
at a distance determined by trial, for correcting the deviation when the ship’s head 
is E. or W. ; and by a mass of unmagnetized iron, at the same level as the compass, 
in the athwart-ship line or in the head-and-stern line according to circumstances 
(usually in the former), also at a distance determined by trial, for correcting the 
deviation when the ship’s head is N.E., 8.E., 8.W. or N.W. 
