74 
View of Mr Barlow's Magnetrcal 
exterior screw and nut, whereby the two plates, and an inter* 
posed plate of wood; of the same size, are compressed strongly 
together ; the board being intended to increase somewhat the 
thickness, without adding much to the weight. It appears, al- 
so, that the two plates thus separated, are more powerful than 
when in immediate contact. The plates are afterwards more 
strongly attached to each other, by three brass pins and nuts. 
The several parts of the plates are represented in Figs. 2. and 
3., and the brass-pin, socket, and pedestal in Fig. 4. ; the lat- 
ter of which shews the whole combined, as in action on ship- 
board. 
Having thus fully described the nature of the operation, let 
us now examine the results that have been obtained in the ves- 
sels before named, and in the Griper, during the recent voyage 
of that vessel to Spitzbcrgen, in which we shall avail ourselves 
of the information contained in Mr Barlow's ce .Report, address- 
ed to my Lords Commissioners of the Admiralty," as we have, 
in the preceding, been indebted to his “ Essay on Magnetic 
Attractions," to which the above report now forms an appendix. 
The first experiments, Avith the correcting plate, were made 
on board his Majesty's ship Leven, which sailed under the com- 
mand of Captain Bartholomew, in April 1820, to the Avestern 
coast of A frica, but returned the following year, under the com- 
mand of Captain Baldey, in consequence of the death of the 
former officer. From Captain Baldey, and from the other officers 
of the vessel, Mr Barlow received a very extensive series of ex- 
periments, made Avith and Avithout the plate, to determine the 
variations of the needle in those seas ; which were accompanied 
by a letter from the commander, speaking in the highest terms 
of the efficacy of the method proposed. We cannot allow our- 
selves to transcribe these experiments at length, but the folloAV- 
ing observations on them by the author, Avill sufficiently explain 
the nature of the results. 
After giving the tabulated series of the experiments alluded 
to, Mr Barlow observes, that “ the best test Ave have of estima- 
ting the accuracy of the corrected variations, or the efficacy of the 
plate for this determination, is by comparing those variations 
with each other which were made on the same, or on subsequent 
days, while the latitude and longitude remained nearly the same; 
