INTRODUCTION. 
339 
by the Admiralty ; therefore I do not enter into further details on this point, except 
to suggest that you should be particularly careful that the ship’s boats, davits, &c. 
are all in the positions they will occupy at sea ; and that it will be quite sufficient 
for your purpose that the deviation should be tried on the sixteen principal points of 
the compass, instead of on thirty-two, as is sometimes done. 
“2. Whilst engaged with the standard compass, have a second compass, of which 
the compass error (meaning thereby the index error) is known, placed in the gimball 
table of your Fox, and observe generally (by means of the lubber-line) whether the 
effect of the ship’s iron is nearly the same at the two positions, viz. at the position of 
the standard compass and at that of the Fox. Observe particularly whether the 
points of no deviation are the same. It simplifies matters greatly that they should 
be so, and that at both positions the points of no deviation should be nearly the north 
and south points. This they will most probably be in a vessel which will not have 
much iron near either position ; but it will be advantageous, when first choosing the 
positions, to try roughly, — by means of a couple of compasses, one in the proposed 
position of the standard compass, and the other in that of the Fox, — whether they 
point alike when the ship’s head is either north or south. By interchanging the 
compasses in these positions, you will prevent any deception which might arise from 
compass errors. 
“The observations which have been described will give you the value of the con- 
stants a and b , for the corrections of all the declinations observed on board throughout 
the voyage, and you will probably find that they will give you work enough for one 
day. 
“3. I shall suppose therefore that you take a second day for the determination of 
the four constants at the position of the Fox. For this you will require the inclina- 
tion and intensity with the ship’s head on the same sixteen points as before, employ- 
ing a deflector for the intensity on this occasion, in preference to weights, as more 
convenient. You will find of course that the points of no deviation with the compass 
become the points of extreme deviation of the inclination and intensity ; for conve- 
nience I shall suppose them north and south points. Having completed the obser- 
vations with the Fox, remove it and observe the horizontal intensity with the head 
successively north, east, south and west, and north again*, placing the apparatus for 
the horizontal intensity on the gimball stand of the Fox. This will give you a and 
b for that position more satisfactorily than the observations of the Fox ; from these 
latter, with the shore observations, you will have c and d. 
“The formulae applicable to all the proceedings which have been described, will be 
found in Mr. Smith’s Memorandum in No. V. of the Contributions to Terrestrial 
Magnetism. But besides the induced magnetism to which these formulae refer, the 
* “ These are compass points, the compass being supposed in strictness to be placed on the spot of the gim- 
ball table ; if a compass placed at this spot has been found to agree with the standard compass, the latter 
gives directly the required azimuth of the ship’s head.” 
