76 
View of Mr Barlow's Magnetlcal 
as correct. By allowing the 7° which we had found subtrac- 
tive from the course, our latitude was by reckoning 38° 41' N. 
and long. 11° 02' W., which agree with observation as closely as 
we can ever expect to do under any circumstances.” 
Thus far, then, the experiments were found fully to answer 
their intended purpose, but an important question was still to 
be decided. Captain Flinders had observed, that, with an equal 
north and south dip, he found an equal quantity of local devi- 
ation ; but in a contrary direction, the north end of the 
needle, in one instance, and the south in another, having been 
drawn forwards by the action of the vessel, it was, there- 
fore, of the highest importance, to ascertain how far the power 
of the plate was competent to meet this strongly marked differ- 
ence in the action of the ship. This was the point left for the 
decision of Captain Basil Hall, in his voyage in the Conway, 
round Cape Horn, to the western coast of America, We can- 
not do better, in describing these experiments, than by follow- 
ing our author’s plan in transcribing Captain Hall’s letter, writ- 
ten on the return of the vessel in 1822. 
44 In practice,” says this officer, 44 the following is the me- 
thod we pursued.” 
44 A set, or several sets of azimuths, were taken without the 
plate, then another set or sets with the plate affixed ; the ship’s 
head, and all other circumstances remaining the same. The va- 
riation of the compass was then computed from each of these 
observations : now, that variation resulting from the first obser- 
vations, was affected simply by the local attractions of the ship, 
and may be termed the deviated deviation : that resulting from 
the azimuths, when the plate was affixed, by an action twice as 
great, viz. first, by the ship, and, next, by the plate, may be 
termed the doable deviated variation. The difference between 
these variations is the amount of the local attraction or the devi- 
ation, and this applied to the deviated variation gives the correct 
magnetic variation. 
44 It is easy to see how this correction is to be applied, by 
merely observing whether the north end of the needle has been 
drawn to the west or to the east, by the application of the plate, 
and considering that the ship’s attraction must have had a simi- 
lar effect on the needle* 
