84 
View of Mr Barlow’s Magnetical 
work longer, that is in higher latitudes, than it would otherwise 
do. This was observed as an experimental fact by Lieutenant 
Foster, while the vessel was on the coast of Greenland, where it 
was obvious, that when the plate was removed the needle was 
wholly inactive, but as soon as the plate was replaced, the needle 
again became serviceable. At this time the cause of this im- 
portant effect of the plate was not known ; but on the return 
of the ship, Lieutenant Foster having attentively reconsidered 
the subject, demonstrated it to be the necessary consequence 
of the common principle of the composition and resolution of 
forces ; a discovery in the highest degree creditable to the acute- 
ness and ingenuity of this rising young officer. On the coast of 
Greenland, the effect of the local attraction of the vessel amount- 
ed to 45° at east and west, so that the action of the iron at this 
time on the needle was equal to that of the earth ; and the di- 
rection which the needle would at any time take up, was in the 
diagonal of the parallelograms described on lines representing 
these two equal forces; ^nd the intensity of the same would, in 
like manner, be denoted by the diagonal of that parallelogram. 
Therefore, with the ship’s head towards the north, the intensity 
would be greater than with the earth alone; with the ship’s head at 
east or west, the intensity would be greater also in the ratio of 
to 1, but when the bead was towards the south, the iron of the 
vessel being opposed in power to that of the earth, the action of the 
needle is destroyed, and is wholly inactive and unserviceable for 
the purposes of navigation. Thus, let NE (Fig. 6. PI. 1 1.) repre- 
sent the force which the earth exercises on the needle, and NS 
that belonging* to the iron of the vessel, and NL the resultant of 
the two ; it is obvious, after the angle SNE exceeds a certain 
quantity, that the resultant NL is less than either of the single 
forces, and which soon becomes too small to bring the needle 
home to its proper direction. But by opposing to the force 
NS another equal force due to the plate, as NP (Fig. 7.), then 
the resultant of the three forces is the single terrestrial force 
NE, and the needle will be as free to move as if no iron were in 
its vicinity. 
This admirable property of the correcting plate leads Lieu- 
tenant Foster to form the most sanguine expectations that, in 
the ensuing voyage, he will have the satisfaction to see the com- 
