182 
LINES OF MAGNETIC DECLINATION IN THE ATLANTIC. 
duced magnetism, giving the corrections which, on that hypothesis, and on the further 
supposition that the changes in the induced magnetism of the ship were simultaneous 
with those of terrestrial magnetism, should have corresponded with the disturbance 
on each of the thirty-two points of the compass under every degree of the terrestrial 
dip. On examining, by means of this table, the number of degrees of dip by which 
it was necessary to go back from the dip at the place of observation, in order to 
obtain from the table corrections corresponding to the disturbances at the stations 
where the ship was subsequently swung, I find that at Port Praya, where the dip was 
H-45° 32', and after a passage of thirty-six days from the British Channel, where the 
dip was about +69°, that the ship’s magnetism, instead of corresponding to a dip of 
+ 45° 32', did in fact correspond to a dip of about +51°^ ; the arrear being about 6°. 
At St. Helena, where the dip was about —20°, and where the ship had arrived after 
a passage of about seventy-nine days from Port Praya, during which she had passed 
from north into south dip, the arrear was between thirty and forty degrees — the 
tabular corrections for 20° north dip corresponding more nearly with the differences 
of the azimuths observed at St. Helena with the ship’s head on different points, than 
did the tabular corrections for 20° south dip ; so that the effect of the employment of 
the latter would manifestly have been to have increased the evil which they were in- 
tended to correct. 
At the Cape of Good Hope, where the dip was —53°, and after an interval of thirty- 
eight days from her departure from St. Helena, the arrear appears to have been about 
twelve degrees. 
At Kerguelen Island, where the ship arrived after a passage of forty days from 
the Cape, but where she remained in harbour about fifty days before the disturbance 
experiments were made, the tabular corrections had overtaken the terrestrial dip, 
although the latter had increased from — 53° at the Cape to —70° at Kerguelen Island ; 
above one hundred days had elapsed between the experiments in — 53° and those 
in —70° of dip, of which less than half the number were occupied in making the 
passage from the dip of —53° to that of —70°, and the rest were passed at the 
anchorage in —70°. From Kerguelen Island the Erebus proceeded to Hobarton, 
where the dip was the same, within a degree, as at Kerguelen Island, being —70° 40', 
and where, as I have already stated, the disturbances were found to be, both in kind 
and amount, very nearly such as might have been computed beforehand from the 
observations in the Thames, by the formulae which apply to induced magnetism sus- 
ceptible of instantaneous change, the tabular corrections compensating the disturb- 
ances within the limits of the usual errors of observation. 
From Hobarton the Erebus proceeded, in November 1840, to the high geographical 
latitudes of the southern hemisphere, and remained for some months in south dips 
much exceeding that at Hobarton, to which station she returned in April 1841. On 
the 29th of June 1841, being about eleven weeks after her arrival in harbour, observa- 
tions were made on the disturbances of the compass on each of the thirty-two points. 
