LIEUT.- COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
93 
The correspondence in the value of the constants obtained from the observations 
at Hobarton and Port Louis, being the commencing and concluding stations of the 
voyage now under consideration, is fully as good as could be desired ; and a table 
formed from them has been employed for the correction of the observations made 
between Hobarton and the Bay of Islands, and during the return of the Expedition 
from the high latitudes to the Falkland Islands commencing with the 1st of March 
1 842. In those portions of the voyage the ship was passing from the higher to the lower 
magnetic latitudes, in which circumstance they corresponded with the observations at 
Hobarton and Port Louis, which were both made on the return from the vicinity of 
the magnetic pole. But if we attempt to apply the same table to the observations 
made under the reverse circumstances, namely, when the ship was passing from the 
lower to the higher latitudes (and such was the case with the greater part of the 
observations which we have to correct in the present voyage), we find that the tabular 
numbers, where the N. and S. points are approached, furnish a decided over compen- 
sation. On days when observations have been made at or near the N. and S. points, 
if we seek in the table for the corrections which should bring the results in accord 
with each other, we find that the corrections which will do so belong to a dip which 
is always some degrees less than the true terrestrial dip. It appeared desirable, there- 
fore, if possible, to form a table for the correction of the observations of this portion 
of the voyage, derived from those observations themselves. Fortunately we have a 
better opportunity of doing this than might have been anticipated. The progress of 
the Expedition was so much impeded by ice in the early part of January 1842, that 
from the 6th to the 16th inclusive, the Erebus was the whole time between the lati- 
tudes of — 65° 54' and —66° 14', and between the longitudes of 204° 33' and 202° 02' ; 
the weather and all other circumstances being favourable, the inclination was 
observed in the course of those eleven days with the ship’s head on seventeen different 
points of the compass, sufficiently distributed, and particularly towards the north 
points and south points, where the effect of the ship’s attraction is greatest, and is in 
opposite directions. From the observations at north and south it is not difficult to 
obtain an approximate value of a , which should bring the corrected results at those 
points into accord. The value thus obtained is about + - 023. I have collected the 
observations during the period referred to into the following table, taking, for the 
sake of simplicity, only those observations which were made by the direct method, 
which, however, comprises by far the greater part of the observations of that period. 
I have then computed the corrections, first, with the values of the constants, such as 
they are given by the observations made for their determination at Hobarton and the 
Falkland Islands (being the commencement and close of the voyage), viz. a— -p028 ; 
b— +‘984 ; c=-{-'015 and d = 1 ; and second, with u=-|- , 023, b, c and d, as before; 
and have placed the two series of corrected results in the table, with columns show- 
ing in both cases the difference of the corrected result, on each point, from the mean 
result. A comparison of those columns seems conclusive in favour of the application 
