1823.] Prof . Barfow on Magnetic Attractions. 455 



remain only while the t\?o bodies maintain their respective 

 situations. 



" 4. The quantity of action thus impressed upon the iron body 

 will depend, jirst, upon the intensity of the exciting magnet ; 

 secondly, upon the capacity of the quiescent body for magnetism, 

 or the quantity of those fluids contained in it; and, thirdly, 

 upon the cohesive power of the iron, which latter quality deter- 

 mines the depth to which the exciting magnet is able to disen- 

 gage the two fluids. 



" The above embraces every case ; viz. of any magnet, natu- 

 ral or artificial, developing the magnetism in any given iron 

 body ; but in that to which our attention will be principally 

 directed, namely, the displacement occasioned by the magnetic 

 action of the earth on spheres of iron, we shall find more limited 

 in its results, and more susceptible of correct mathematical 

 investigation. 



" 5. In this case, for instance, we may suppose the action to 

 take place on every particle of the mass in lines parallel to each 

 other, and corresponding with the direction of the dipping 

 needle ; also that every particle is at the same distance from the 

 centre of the disturbing force, and consequently that the dis- 

 placement in each particle is equal also ; conditions which 

 throw great facilities into the analytical investigation of the 

 laws of action. 



" G. For the sake of illustration, let A B C D, fig. 9 (PI. XX), re- 

 present a sphere of iron in its non-magnetic, or quiescent state, and 

 let C M be the line in which the terrestrial magnetism is exerted 

 from a centre of action, M, which is at such a distance that the 

 diameter of the sphere is inconsiderable in comparison with it; 

 then every particle on its surface, and to a certain distance 

 within it, will be acted upon by equal powers, and in directions 

 parallel to each other; whereby the fluids in the quiescent body, 

 before in a state of combination, will be separated in each par- 

 ticle ; and the two fluids may now, therefore, be conceived to 

 form two spherical shells, A e B d, A e' B d' , whose centre of 

 action will be in c, c', their distance from each other being 

 greater or less, according to the circumstances stated in No. 4. 



" 7. Therefore, in computing the action of such a mass of iron 

 in its temporary state of magnetism upon a distant particle of 

 magnetic fluid, we may refer it to those centres ; we shall also 

 assume, that the law of action in this, as in all other cases of 

 central action, is inversely as the square of the distance." 



The limits to which we are necessarily restricted will not 

 allow us to proceed with our author to the mathematical deduc- 

 tion from the foregoing hypothesis of the laws which he had 

 previously drawn from experiment ; nor to his computations 

 respecting the horizontal and dipping needles ; suffice it to say, 

 that the approximative agreement of their results with actual 

 observation, is of a very satisfactory nature. We are likewise 

 compelled to pass over the second, third, and fourth sections. 



