500 ON PHYSICAL LINES OF FORCE. 



The velocity of light in air, as determined by M. Fizeau*, is 70,843 leagues 

 per second (25 leagues to a degree) which gives 



7=314,858,000,000 millimetres 

 = 195,647 miles per second ...................... (137)- 



The velocity of transverse undulations in our hypothetical medium, calculated 

 from the electro-magnetic experiments of MM. Kohlrausch and Weber, agrees so 

 exactly with the velocity of light calculated from the optical experiments of 

 M. Fizeau, that we can scarcely avoid the inference that light consists in ll<- 

 transverse undulations of the same medium which is the cause of electric and 

 magnetic phenomena. 



PROP. XVII. To find the electric capacity of a Leyden jar composed of 

 any given dielectric placed between two conducting surfaces. 



Let the electric tensions or potentials of the two surfaces be , and ,. 

 Let <S* be the area of each surface, and 6 the distance between them, and let 

 e and e be the quantities of electricity on each surface ; then the capacity 



Within the dielectric we have the variation of perpendicular to the surface 



e 



Beyond either surface this variation is zero. 



Hence by (115) applied at the surface, the electricity on unit of area is 



(139); 



and we deduce the whole capacity of the apparatus, 



8 



so 



that the quantity of electricity required to bring the one surface to a 



* Comptes Rendus, Vol. MIX. (1849), p. 90. In Galbraith and Haughton's Manual of Astronomy, 

 M. Fizeau's result is stated at 169,944 geographical miles of 1000 fathoms, which gives 193,118 

 wtatute miles; the value deduced from aberration is 192,000 miles. 



