72 View of Mr Barlow's Magnetical 
nature, and which, although simple in operation, was so inti- 
mately connected with one of the most intricate branches of na- 
tural philosophy. But, before entering farther into the results 
of these experiments, it will be proper to state a few particulars 
relative to the method of fixing the plate in its proper situation, 
and the construction of the plate itself. 
In order to ascertain the situation for the plate, a place must 
first be selected, by the captain, for the azimuth or regulating 
compass to be fixed in for observation during the period of the 
ship being in commission. It will then be necessary to ascer- 
tain the local attraction of the vessel, which may be done in the 
following manner. The ship being moored, or lying with a 
short scope of cable, must have anchors so arranged, as to admit 
of her head being directed to each point of the compass succes- 
sively, and there steadied, whilst the bearing of a remote object 
is taken (the more distant the better), to avoid the parallax, 
which would otherwise alfect the observations. It will then be 
found, that the bearings thus observed differ from each other, 
according to the attractive power of the vessel, from 6 or 8 to 
26 or 28 degrees ; a difference which is caused by the iron of the 
ship attracting the needle out of its proper direction to the east- 
ward, with the ship's head east, and to the westward with the 
head to the west. On examining these several bearings, there 
will be found two, at opposite points of the compass, that will 
nearly agree with each other, the mean of which must be ac- 
counted the true magnetic bearing of the object ; and these 
points will also indicate the line of no attraction in the vessel, 
and which will generally be found nearly fore and aft : in this 
line the plate is ultimately to be fixed. By comparing the cor- 
rect magnetic bearing, as before found, with the observed bear- 
ing at the several points, the amount of the local attraction, at 
each point, will be ascertained. 
It now remains to determine the position of the plate, which 
hitherto has been a matter of experiment, by taking a pedestal 
or compass-stand on shore, and by trying different situations for 
it, and turning it round, till the same deviations were produced, 
by the plate at each point, as had been already observed in the 
vessel ; but, in future, this will be more readily done, by means 
of a small printed table, which Mr Barlow intends to supply 
