132 
MAJOR SABINE ON TERRESTRIAL MAGNETISM. 
In three of the comparisons the differences are inconsiderable, amounting only to 
a very few minutes ; in the fourth it amounted in one observation nearly to a degree. 
It must be noticed, however, that it is more difficult to get true dip results on hoard 
a ship at anchor, ‘particularly where there is no tide, than when under sail, because 
the ship’s head cannot be kept so steadily on one point ; and by the frequent varia- 
tion in the direction of her head relatively to the magnetic meridian, the observed dip 
is rendered greater than the true ; the error hence arising being greatest where the 
dip is least. Thus in all the four comparisons the dips observed on board are always 
somewhat in excess of those observed on shore ; and it is probable, that the single 
difference which approached a degree in amount was occasioned by an accident of 
this nature. In the packet Opossum, in which more than the half of Lieut. Sulivan’s 
sea observations were made, the difference is less than in either of the Queen’s ships, 
and is much within the limits of ordinary variations in successive observations made 
on shore. The ship’s head was always placed in these comparisons as nearly as pos- 
sible on the point of the compass which corresponded with the course in that part of 
the voyage ; and, at sea, the observations were always made with the ship’s head on 
her course, except in one instance in the Arrow, when it was varied for the sake of 
experiment. 
The comparison of the intensity results on board and on shore, in similar trials, 
present an even more satisfactory accord ; and this consists with a remark made by 
Lieut. Sulivan, that by shifting the place of observation to different parts of the ship, 
the dip might be made to vary nearly a degree, but that in such cases the angle de- 
noting the intensity remained nearly or wholly unaltered. The comparisons of the 
intensity were made only on three occasions, viz. 
Rio de Janeiro, September 4th, 1838. 
On shore Intensity 0-854 
In the Arrow, head S. by E 0-855 
Rio de Janeiro, April 4th, 1839. 
On shore . 0'854 
In the Stag, head N. by W 0-853 
Pernambuco, May 7th, 1839. 
On shore 0-899 
In the Opossum, head N.N.E 0-900 
The four observations at Rio, viz. in the Arrow, in the Stag, and at two different pe- 
riods on shore, may be regarded as identical ; and the two observations at Pernam- 
buco are not less accordant. 
Lieut. Sultvan’s dip results at sea are distinguished in the maps by inclined figures, 
placed on the right side of the mark designating the geographical position corre- 
