94, .REPORT—1862. 
From Mr. Airy’s observations we derive the following values for the 
coefficients :— 
Bers) ogBe ce | D. E. i 
ie) 1 ° ly fe} U 7 oe Oo TF 
‘Rainbow,’ station 1 | + 0 40] — 50 36/— 11 4/+123|+0 38] -984 
Ff 5 +'0 385|— 18 45 | — 12 57| + 230) +02) -9735 
5 . 3/+ 0 42| — 15 46| — 10 39} +3 07/—0 2/1-003 
: ap 4\4.0 5|—.8..5|/— 933) +3 261 +0) 2)" “939 
‘Tronsides’ ......| +0 9]— 2416] + 20 59] 4+ 2 06/4016; -908 
The most remarkable features in the deviations of these ships are the very 
small amount of the quadrantal deviation, and also in the ‘ Rainbow’ the 
small diminution of the horizontal force. 
These features led Mr. Airy to the conclusion that the amount of in- 
duced magnetism was small, and that nearly the whole of the semicircular 
deviation was caused by permanent magnetism. That this was the case as 
regards the coefficient C there can be no doubt ; but as regards the coefficient B 
the case is different, as any part of it may have arisen from the induction in 
vertical masses of iron before or abaft the compass. 
These results, and the conclusions which Mr. Airy drew as to the amount 
of permanent magnetism, were the foundation of his well-known method of 
correcting the deviations by means of magnets and soft iron, which has 
been so extensively practised in the mercantile marine. 
Another remark may be made on the results. One of the most import- 
ant conclusions which have been drawn from the numerous observations 
which have been made on the deviation of iron-built vessels is, that, in 
a well-selected place for the standard compass, the semicircular devi- 
ation depends on the position of the ship in building, the magnetism which 
would be assumed if the iron were soft being then, by the process of 
hammering, fixed in the vessel, and a character then impressed which the 
ship never afterwards loses,—the general result being that the north point 
of the compass is attracted to that part of the ship which was south m 
building, so that +B indicates a ship built head south, —B a ship built head 
north of the (magnetic) east and west line, +C a ship built head east, and 
—C a ship built head west of the magnetic meridian, With our present 
knowledge, we should have little hesitation in drawing the conclusion from 
Mr. Airy’s observations, that the ‘Rainbow’ was built with her head not 
far from N.W., and the ‘Ironsides’ with her head not far from N.E. At 
that time, however, the connexion between the direction of building and the 
semicircular deviation was unsuspected*, and the direction in which those ships 
* To this there is one exception, which deserves to be recorded. In the year 1835, 
Captain Johnson made elaborate experiments on the magnetism of the iron steam-vessel 
‘Garry Owen,’ the results of which are contained in a paper in the Phil. Trans. for 1836, 
. 267, Captain Johnson ascertained, from observations made on a needle on shore, that 
the ‘Garry Owen’ acted as a permanent magnet, her head repelling, and her stern 
attracting, the north end of the needle; and he says, p. 285 :—“‘ As, in the construction of 
iron vessels, hammering the numerous rivets might elicit magnetic influences, it would be 
well to note, by compass, the direction of their heads and sterns when building, with a 
view of ascertaining whether (in combination with the former circumstances) any distinct 
magnetic properties indicated by those parts are due to the line of direction of the vessel 
with respect to the magnetic meridian.” 
«The head of the ‘ Garry Owen,’ when building, was W.N.W.” 
It may seem singular that Captain Johnson did not observe how nearly this direction 
