ON PHYSICAL LINES OF FORCE. 499 



traverse the section of this current in one second, these units being such that 

 any two of them, placed at unit of distance, repel each other with unit of force. 



We may suppose either that E units of positive electricity move in the 

 positive direction through the wire, or that E units of negative electricity move 

 in the negative direction, or, thirdly, that \E units of positive electricity move 

 in the positive direction, while \E units of negative electricity move in the 

 negative direction at the same time. 



The last is the supposition on which MM. "Weber and Kohlrausch* proceed, 

 who have found 



} 155,870,000,000 (130), 



/ 



the unit of length being the millimetre, and that of time being one second, 

 whence 



j = 310,740,000,000 (131). 



PROP. XVI. To find the rate of propagation of transverse vibrations 

 through the elastic medium of which the cells are composed, on the suppo- 

 sition that its elasticity is due entirely to forces acting between pairs of particles. 



By the ordinary method of investigation we know that 



where m is the coefficient of transverse elasticity, and p is the density. By 

 referring to the equations of Part I., it will be seen that if p is the density 

 of the matter of the vortices, and \L is the " coefficient of magnetic induction," 



\L = TT P (133); 



whence mn= Vp (134); 



and by (108), E= Vjji, (135). 



In air or vacuum ^=1, and therefore 

 V=E 



= 310,740,000,000 millimetres per second 



= 193,088 miles per second 



* AbhamHungen der Ki/nig. Sachsischen Geselkcliaft, Vol. m. (1857), p. 260. 



(136). 



632 



