LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
117 * 
was so obliging- as to favour me, printed in the last number of these Contributions, I 
avail myself of this oportunity of giving it an early circulation. 
“The apparent changes in the values of the constants a , b, c and d, in the Erebus 
and Terror (Contributions, No. V., p. 153), seem to show that those vessels had 
an appreciable quantity of magnetism, which was so far permanent, as to retain for 
a considerable time traces of the inductive force to which they had been exposed, 
and perhaps some strictly permanent magnetism. It seems, therefore, desirable to 
introduce into the expressions in the memorandum printed at p. 147 of Contribution 
No. V., terms which will represent such forces. 
“Suppose, then, as in the memorandum, that <p represents the total magnetic 
force of the earth at the place of observation, 0 the inclination, £ the azimuth of the 
ship’s head, reckoning from N. to W., and that <p' } O', Z) represent the values of the 
same quantities shown by an instrument at a fixed position in the vessel, and affected 
by the attraction of the iron in the vessel ; and let P, Q, R represent the attraction 
of the permanent magnetism in the vessel to the bow, to the starboard side, and ver- 
tically downwards. The fundamental equations of the former memorandum become 
by the introduction of these terms, 
<p' cos O' cos [A' cos 0 cos ^ + B cos 0 sin £ + C sin d] -f- P 
<p' cos O' sin %=<p [D cos 0 cos ^ + E' cos 0 sin £-(- F sin d] + Q 
d sin 0' = <p [G cos 0 cos H cos 0 sin £ + K' sin 0\ + R. 
“ In these equations A', B, C, D, E', F, G, H and K' are constants depending on the 
distribution of the soft iron in the ship, and perhaps on the temperature and other 
circumstances. 
“ If we suppose, as before, that the soft iron is symmetrically disposed, the equa- 
tions (1.) (2.) and (3.) of the former memorandum become. 
d cos 0' cos 
A'cfi cos 0 
cos Z, + a tan 0 
P 
A'<$> cos 0 
d cos 0' sin ^ , . „ ( Q, 
A'$cos0 Sin ^ ‘ A'<f>cos0 
d sin 0 f 
A'<f> cos 0 
= c cos Z, + d tan 0 -J- 
R 
A'$ cos 0 
(I.) 
( 2 .) 
( 3 -) 
“ Let H represent the horizontal force = <p cos 0, H' the affected horizontal force 
= <p' cos O', and let a tan 0 + xrjj=L, = M, and d tan 0 -f- = N. The last 
equations become 
cos £'= cos £ + L (In.) 
sin Z,'=b sin £ + M (2 a.) 
-yjj- = ccos^-f N (3 a.) 
