205 
of his experiments by Lieut. Foster. 
the compass by means of an iron plate. Having observed 
the effects that were produced on the needle by the rotation 
of an iron plate previous to the sailing of the Leven and 
Barracouta, in the spring of 1822, these vessels being fur- 
nished with correcting plates, I communicated the discovery 
to Mr. Barlow, and stated that probably the correction 
might be sensibly affected by it, unless rotation, in applying 
the plate, were prevented, by having the pin so formed that 
the plate could only be slid on. The preceding observations 
prove clearly the importance of attending to this, especially 
in high magnetic latitudes, should circumstances require the 
removal and replacing of the plate, since there can be no 
doubt, from the magnitude of the deviations arising from 
rotation, observed by Mr, Foster, that if in replacing the 
plate, it were made to revolve, although it might be in pre- 
cisely the same situation as before, its magnetism would be 
so materially changed, that the attraction of the ship would 
no longer be corrected by it. Should such a circumstance 
take place, it may be proper to mention that the plate would 
be restored nearly to its original state, by allowing it to 
remain for some time with its plane in that of the magnetic 
equator. 
S. H. CHRISTIE. 
Royal Military Academy, 
10th January, 1826. 
