340 MAGNETIC SURVEY OF A PART OF THE SOUTHERN HEMISPHERE. 
iron of a ship is found sometimes to exercise upon its compasses a magnetic influence 
of a distinct character, to which it may become in some instances desirable to give a 
separate consideration. This influence may be either from permanent magnetism 
strictly so called, or from a polarity which is temporarily retained, and undergoes 
alterations consequent upon changes in the inducing action in which it originated, 
but following after them at a greater or less interval of time. This additional mag- 
netic force may be represented by additional symbols, P, Q and R, i. e. the force re- 
solved along the principal section of the ship, transversely to it, and in the vertical 
direction. 
“ The alterations which the introduction of this force makes in Mr. Smith’s formula 
are stated in a second memorandum now printing in No. VI. of the Contributions, a 
copy of which will be in your hands before you sail. 
“ This memorandum furnishes equations by which all the constants may be deter- 
mined by observations in different magnetic latitudes, — -of the horizontal force on 
the four principal points, — and of the dip on the tivo principal, together with the dip 
and horizontal force observed on shore or on the ice. These are part of the obser- 
vations already directed. 
“4. The observations described in No. 3 must be repeated on the return to the 
Cape at the conclusion of the voyage, before any change has been made in the iron 
of the ship. If polarity due to the inducing action of a higher magnetic latitude has 
been retained, the observations on the return will be found to differ from those made 
before you sailed. If the disturbing influence of the ship’s iron be solely the effect 
either of instantly induced magnetism, or of permanent magnetism strictly so called, 
the observations will agree with those made before the departure of the vessel. 
“ 5. If in the course of the voyage you should anchor in any port in a high lati- 
tude, at Enderby’s Land for example, or at the Adelie Land of d’Urville, it will be 
extremely desirable to repeat the same observations. Whenever a choice exists be- 
tween the shore and fixed ice, as a place for observation out of the influence of the 
ship’s iron, always prefer the fixed ice. 
“6. The approximate value of a , the most important of the constants, may be ob- 
tained on board at any time during the voyage when the weather is sufficiently 
favourable, by azimuths at the north or south points and at the east or west points 
for the position of the standard compass, and by the horizontal intensity observed 
on the north and south points for the position of the Fox. If Hansteen’s needles are 
used for the latter purpose, and n,s, be the number of vibrations at north and south 
in a certain time, commencing at the same arc, and performed in a nearly uniform 
IX 
temperature, then — — tan and cos 2 X = a tan 0 ; also if A = the deviation 
when £'=90°, 
A=90°-2A. 
“ 7* The horizontal intensity at the north and south points should be observed on 
