96 REPORT—1862. 
these there is the great difference that the force which gives rise to @ and to 
the first part of 33 ceases to operate, or at least ceases to operate in the same 
direction, the moment the ship has been launched, and has her head directed 
to different points of the compass, while the force causing the other part of 33 
continues to act in its original direction as long as the ship remains in and 
near its original geographical position. 
€, whatever its magnitude, may therefore be expected to diminish rapidly 
after launching, and until the originally impressed magnetism reaches (as 
it appears ultimately, and in fact after no long period, to do) the limit 
beyond which sensible change does not proceed, and on a change of latitude 
it will vary inversely as the horizontal force. 38, on the other hand, although 
it may change considerably after launching, if the ship has been built north 
or south, will, if the ship has been built east and west, remain unchanged. 
On the other hand, on a change of magnetic latitude, while the effect of the 
subpermanent magnetism induced by the horizontal force will vary inversely 
as the horizontal force, that part which has been caused by the original ver- 
tical magnetism may change more rapidly from the change in the inducing 
cause, and the remaining part, or the transient vertically induced magnetism, 
will in its effect vary as the tangent of the dip. 
The combination of these several causes renders the discovery of the true 
source of the 3 a matter of great difficulty, even when observations have 
been made in several different latitudes. 
That the distribution of the permanent magnetism of iron ships is 
principally owing to their position in building appears to have been first 
strongly insisted on by Dr. Scoresby in the 4th Part of his magnetical 
investigations published in 1852. The great importance of the service thus 
rendered by Dr. Scoresby cannot be over-estimated. Dr. Scoresby also en- 
deavoured to investigate the changes which the subpermanent magnetism of 
a ship undergoes on a change of magnetic latitude. He did so, however, with 
very insufficient materials, and it appears to us (as one of us has endeavoured 
to point out with greater detail in the introduction to the ‘Journal of a 
Voyage of Magnetic Research’), without having sufficient regard to the amount 
of transient vertically induced magnetism which acts or may act as a cause 
of semicircular deviation. 
At the meeting of the British Association. at Liverpool in 1854, Dr. 
Scoresby brought the subject of the change of a ship’s magnetism promi- 
nently before the Association, in a paper on the loss of the ship ‘Tayleur’ 
and the changes of the compasses of iron ships. The discussion so occa- 
sioned gave rise to the formation of the Liverpool Compass Committee, whose 
valuable Reports are one of the special subjects on which we are commis- 
sioned to report, and also to Dr. Scoresby’s voyage in the ‘ Royal Charter’ 
for the purpose of observing the changes which take place in the magnetism 
of an iron ship on a change of magnetic latitude. To these we now address 
ourselves. 
The Liverpool Compass Committee have had the assistance throughout of a 
most able Secretary, Mr. W. W. Rundell, who has brought to the subject an 
amount of practical and scientific knowledge, combined with industry and 
zeal, which have given to the three Reports which have been published the 
highest possible value. 
The first Report bears date the 5th of February, 1856, shortly before 
Dr. Scoresby sailed in the ‘ Royal Charter.’ The second Report bears date 
February, 1857, and embodies the principal results of the observations in the 
‘Royal Charter.’ The third Report bears date the 13th of February, 1861. 
