Astronomical and Nautical Collections. 325 
sequel of this memoir, that this difference would not have any sen- 
sible effect in the exterior magnetic action of the substances; so 
that it would not serve to explain the difference of the actions 
which they exert, in the same circumstances, on magnets placed in 
their neighbourhood. 
"If now we consider the case of a magnetized body of indetermi- 
nate form and dimensions, we must attend te the lines or directionsin 
which the separation of the two fluids takes place throughout its sub- 
stance, and in which they are arranged alternately, as in the needle 
which has been taken for an example. ‘These lines will in general 
be curves depending on the form of the body, and on the external 
forces which act on the two fluids: they may be termed lines of 
magnetization, and we may call the minute parts, of which they are 
composed, magnetic elements, each containing the boreal and 
austral fluids in equal quantities. Thus, in each particular pro- 
blem, we shall have to determine, for each point of the body to be 
considered, the direction of the line of magnetization, [or polarity, ] 
and the action of the magnetic element on any other point given in 
position within or without the body. This action is the difference 
of the forces exercised by the two fluids contained in the element, 
arising from the slight separation of the boreal and austral mole- 
cules in the state of polarity. It is somewhat surprising to see 
that forces depending on distances so small, between the centres to 
which they belong, should be capable of producing mechanical ef- 
fects so manifest, as those which result from magnetic attractions 
and repulsions ; but in fact the result of the action of all the mag- 
netic elements of a magnetized body is a force equivalent to the 
action of a very thin stratum covering the whole surface of the body, 
and formed of the two fluids, the boreal and the austral, occupying 
different parts of it. Now we are well acquainted, in the attractions 
and repulsions of the conductors of electricity, with mechanical 
effects, sometimes very powerful, which are produced by strata of 
fluids of a thickness so inconsiderable as to escape our senses and all 
our means of appreciating it. As to the ultimate magnitude of the 
forces which we are thus required to attribute to cach of the two 
separate portions.of fluid, whether boreal or austral, belonging to 
