Experiments and Discoveries . < 1 
all forms and dimensions ; the centres of action in all cases 
being referred to two points indefinitely near to each other in 
the general centre of attraction of the disturbing mass. 
Let us now see how far the principles thus developed are ap- 
plicable to the correction of the local attraction of vessels. First, 
Since the action of any mass of iron or system of iron bodies 
may be referred to two points indefinitely near to each other in 
the general centre of attraction of such mass or system, it fol- 
lows, that in a ship all the action may be referred to a fixed 
point in the vessel, and that the line joining that point, and the 
pivot of the needle, will be fixed in position, as regards the ship, 
in all parts of the world : and, secondly, Since the whole mag- 
netic power of iron resides only on the surface, it is easy to pro- 
cure a plate of iron of inconsiderable weight, that shall have, 
when placed near the compass, an effect upon the needle equal 
to the great mass of iron in the vessel situated at a greater dis- 
tance ; and if also the centre of attraction of this plate be fixed 
in the line above mentioned, then in whatever direction the ves- 
sel is placed, or in whatever part of the world the observation 
may be made, the plate will always produce a disturbance on the 
needle equal to that of the ship ; so that by obtaining the one 
by observation, the other, that is, the disturbance produced by 
the ship, becomes known. Hence, whatever may be the law by 
which this disturbing power is governed, or whatever may be 
the dip of the needle or the direction of the ship's head, the 
plate and the iron of the vessel attract the needle the same, both 
in quantity and direction, and the one becomes known by the 
observation made on the effect of the other. This was the idea 
first suggested for practice, b} r Mr Barlow, to the Admiralty : 
and, on this principle, the experiments were made by Captain 
Baldey, in the Leven, and by Captain Basil Hall in the Com 
way ; the observations in the latter case having been carried on 
from England below Cape Horn, to the latitude of 61° south ; 
and throughout this great arc of terrestrial latitude, the results 
are the most satisfactory that can be desired. It was indeed a 
fortunate circumstance, that our scientific countryman should be 
at this time placed in command of a vessel bound to so distant 
a port, for we know of no officer in the British service who 
was more likely to do justice to an experiment so novel in its 
