258 Mr. G. W. Hearn on the Composition and 



the corresponding rising of the effect of that force in the glass. 

 M. E. Becquerel does not agree with me in the above expla- 

 nation, but thinks it is due to this ; that the particles of the 

 diamagnetic itself require time to assume their new state. 

 That they may require time is, I think, very probable. I do 

 not know any state the acquiring of which does not need 

 time. This time is however most probably exceedingly small 

 in the instance of diamagnetic bodies ; and that the pheno- 

 mena referred to by me are not due to such a cause, is, I think, 

 shown by two considerations. The first is, that the electro- 

 magnet is well known by other kinds of proof, as the induc- 

 tion of currents, &c, to require time to develope its maximum 

 force due to a given electric current. I have shown that the 

 rotation of the ray must increase during the time the magnet 

 is thus rising; and I found that the power to induce currents 

 exists simultaneously with the increasing rotation. The next 

 consideration is, that when the diamagnetic is submitted, not 

 to the action of an electro-magnet, but of a helix, the rotation 

 does not increase in the same gradual manner as before, but 

 is instantly at a maximum (Experimental Researches, 2195) : 

 hence my reasons for adhering to the explanation I gave in 

 the papers themselves last year. 



But on subjects so new as these, differences of opinion must 

 inevitably arise upon many points; and it is better for the in- 

 quiries themselves that it should be so, for the facts in con- 

 sequence receive a more close investigation. I therefore leave 

 many points of difference between myself and others unnoticed 

 for the present; believing that new and connected facts will 

 rapidly accumulate, and that time, with his powerful aid, will 

 in a very few years give both facts and opinions their right 

 places. 



Royal Institution, August 11, 1846. 



XLII. On the Composition and Resolution of Forces. By 

 G. W. Hearn, Esq., B.A. of Cambridge, and Professor of 

 Mathematics in the Royal Military College, Sandhurst *. 



1. T^QUAL forces are such as being applied to the same 

 -*~^ point in contrary directions make equilibrium. 



2. The resultant of any number of forces acting on a point 

 is a single force, statically equivalent to the whole of them, 

 that is a force such as would produce the same pressure in 

 the same direction. 



3. If two equal forces inclined to each other act on a point, 

 their resultant is in the plane of the two forces, and bisects 



* Communicated by the Author. 



