322 Astronomical and Nautical Collections. 
we ought to consider the magnetism of two different bodies, of pure 
nickel and of soft iron, for example, as the same imponderable sub- 
stance. It is therefore from experiment alone that we can learn 
whether, neglecting the effect of the coercive force, which is very 
small in both cases, the same exterior cause will produce the same 
effect on the magnetic fluid contained in both these metals; or to 
speak more precisely, whether similar and equal needles of iron 
and of nickel, when submitted to the magnetic influence of the 
earth, or of any other magnet, would make ia equal times an equal 
or unequal number of oscillations. Mr. Gay Lussac has been so 
good as to furnish me with an answer to this question, which he 
has obtained by substituting, for the experiment here proposed, 
another not less conclusive, which he considers as more capable of 
an accurate result. 
A magnetized needle, about eight inches in length, was made to 
vibrate horizontally near the direction of the magnetic meridian ; 
and by the action of the earth alone it made 10 oscillations in 131 
seconds : a prismatic bar of soft iron of nearly the same length with 
the needle, nearly 2 of an inch wide, and -4; of an inch thick ina 
vertical direction, was fixed at the distance of two inches below the 
needle, and in the plane of the magnetic meridian: the oscillations 
of the needle were immediately accelerated, so that there were at 
first 10 in 65 seconds, and soon afterwards 10 in 60 seconds ; but 
they did not become more frequent afterwards. The bar of soft 
iron was then exchanged for a similar and equal bar of pure nickel ; 
and the needle made at first 10 oscillations in 78 seconds, and after 
some time 10 in 77 seconds; and when the bar of nickel was re- 
moved, the needle returned very nearly to its original state, making 
10 oscillations only in 130 seconds, by the action of the earth alone. 
The bars of iron and nickel did not exhibit any perceptible mag- 
netism after the operations, so that the coercive force, if it existed 
at all, must have been very weak in them: it might, however, be 
concluded, that it was not absolutely wanting, because the bar 
did not arrive immediately at the maximum of their action on the 
needle; but this circumstance may also have depended on the action 
of their magnetism on that of the needle, which required a certain 
