128 
Terrestrial Magnetism. 
very valuable, and contain an ingenious explanation of 
the cause and mode of action of this deviation, which 
subsequent investigation has proved to be correct. They 
gave rise to a series of experiments which were directed 
to be made in various ships, particularly ships of war, 
where the mass of iron comprised in the guns, shot, &c. 
is considerable, in order to ascertain the amount of local 
attraction or deviation of the compass in each ship 
correctly on every point; and an extensive investigation 
on the subject was afterwards instituted by Professor 
Barlow, of the Royal Military Academy at Woolwich, 
with a view of discovering some principle of computation 
for correcting this dangerous source of error. Flinders 
argued that “ the attractive power of the different bodies 
in the ship, which are capable of affecting the compass, 
is collected into something like a focal point, or centre 
of gravity; that this point is nearly in the centre of the 
ship; and that, moreover, this point is endowed with 
the same kind of attraction as the pole of the hemisphere 
where the ship is.” Mr. Barlow^s idea w r as, that since 
the guns and other iron of a ship must produce exactly 
the same deviation of the needle as a smaller mass of 
iron placed in a similar situation, but as much nearer as 
its mass is smaller, it w'ould be possible to place such a 
body of iron abaft the compass as would exactly counter¬ 
balance the action of the iron before it; and, conse¬ 
quently, leave the needle as free to move as if no such 
action existed. 
Some disadvantages were, however, found to attend 
the placing of this compensating ball, as it required 
constantly shifting for every different position of the ship; 
and he therefore hit upon the following expedient, 
which was found to answer under all circumstances. 
He placed a ball of iron in the same line of direction 
with regard to the compass, as that in which the com- 
