IN WOOD-BUILT AND IRON-BUILT SHIPS. 
75 
In the Trident, at 
. Greenhithe (No. 12). . 
O 1 
... -1-4 11 
Malta (No. 13) . . . 
... -1-3 44 
Greenhithe (No. 24). . 
... -1-3 35 
Rio de Janeiro (No. 25) 
. . . -1-3 54 
In the Vulcan, at . 
. Portsmouth (No. 26) 
. . . -1-3 36 
Simon’s Bay (No. 27) . 
... -1-4 9 
In the Simoom, at 
. Portsmouth (No. 28) 
... -1-4 31 
Simon’s Town (No. 29) 
... -1-4 14 
The general accordance here is extremely good. The petty discordances appear to 
be purely accidental. It is evident, at least, that they are not dependent on the 
geographical locality. Thus at (8) the Piraeus produces the largest and at (11) the 
smallest in the group; at (12) Greenhithe produces the largest and at (24) the 
smallest; at (27) Simon’s Bay produces the larger and at (29) the smaller. Nor 
have I been able to connect these differences with any other law. Regarding their 
accidental character as established, they give a measure of the range of accident in 
these observations, and they show that that in the Spy, though large, is not excessive. 
1 think it therefore certain that this part of the theory is entirely supported by the 
observations; and therefore that this quadrantal deviation may be perfectly neutral- 
ized in all localities by a mass of soft iron placed in the manner described at the 
beginning of this paper, leaving only a deviation which follows accui’ately in every 
place, separately considered, the laws of polar-magnet-deviation, and which therefore 
in every place, separately considered, may be neutralized by the application of 
permanent steel magnets. 
I shall now proceed to consider the deductions from the two elements of polar- 
magnet-deviation, namely the starboard angle of the polar-magnet-force and the 
modulus. For this purpose, I premise the following elements of terrestrial mag- 
netism. The forces are expressed in Gauss’s method, adopting as units the English 
foot and the English grain. For some of the elements I am indebted to the kindness 
of Colonel Sabine : others were obtained from other sources. None were furnished 
to me precisely in the form in which they are here exhibited, and some calculation 
therefore has been required to adapt them to my wants. It is possible that they 
may be affected by trifling inaccuracies. 
Horizontal Force. 
Vertical Force. 
1. Gillingham . . . 
. 3*78 . . 
. . + 9-94 
2. Porto Praya . . . 
. 6*26 . . 
. . -f 6-38 
3. St. Helena .... 
. 5-97 . . 
. . — 1-97 
4. Cape of Good Hope . 
. 4-56 . . 
. . — 6-08 
5. Kerguelen’s Land 
. 3-88 . . 
. . —10*68 
6. Plymouth . . . . 
. 3'82 . . 
. . -1- 9*67 
L 2 
