44 Account of Captain Scoresby’s 
the instrument is put together by screws, it can be easily taken 
to pieces, so as to become exceedingly portable 
In using this instrument for measuring small magnetic attrac- 
tions, and enabling him to present a piece of iron to a compass 
at different times invariably at the same angle and distance, he 
could ascertain, if, by any treatment to which it might in the 
mean time have been subjected, any change whatever had taken 
place in its state as to magnetism. He then observed, that any 
kind of mechanical action upon a bar of iron, produced a change 
in its magnetical state, which, on being fully investigated, was 
found to follow a similar law as that of magnetism of position, 
with regard to the quality of the magnetism produced. Dr 
Gilbert, indeed, two hundred years ago, discovered that iron, 
when hammered in the magnetic meridian, became magnetical, 
in so far as, when made red-hot, and drawn out in this position, 
to be able to conform itself to the magnetic north and south, when 
floated by a piece of cork upon water. But Mr S. went much 
farther. He ascertained, as it was reasonable to expect, that a 
horizontal position in the magnetic meridian was by no means 
the best position for the development of magnetism by percus- 
sion ; but that the position of the dipping needle given to bars 
of iron, when hammered, produced the highest effect. A single 
blow with a hammer, on a bar of soft iron, held vertically, was 
found to be capable of giving it a strong magnetic action on the 
compass, the upper end becoming a south pole, and the lower a 
north pole ; and that, on inverting the bar, another blow was 
found sufficient to change the polarity formerly given to it. But 
one of the most curious and important effects of percussion ob- 
served by Mr S. was found to be this, that a blow, struck up- 
on any part of a bar of iron, while held in the plane of the 
magnetic equator, (which is horizontal E. and W., or with the 
north end elevated about 19 degrees above the horizontal in 
this country), has an invariable tendency to destroy its magne, 
tic action, which it generally does so effectually as to prevent 
its exerting any influence over a compass, when presented to it 
in the same plane of the magnetic equator. 
Mr Scoresby was led to apply this property to some import- 
ant purposes. Previously no other method was known of free- 
* Edinburgh Transactions , vol. ix. p. 244. 
