156 



Dr. J. Hopkinson. 



[Nov. 17, 



recorded on the maps, and he has carried out this work with a care, 

 skill, and patience beyond all praise. The observations have in nearly 

 every case been checked also by myself. Mr. Taylor, the Demon- 

 strator of Astronomy, has been chiefly responsible for looking up the 

 literature and mapping the results, in which he has been aided by 

 Mr. Richards. 



II. " Specific Inductive Capacity." By J. HOPKINSON, M.A., 

 D.Sc, F.R.S. Received October 14, 1887. 



The experiments which are the subject of the present communica- 

 tion were originally undertaken with a view to ascertain whether or 

 not various methods of determination would give the same values to 

 the specific inductive capacities of dielectrics. The programme was 

 subsequently narrowed, as there appeared to be no evidence of serious 

 discrepancy by existing methods. 



In most cases the method of experiment has been a modification of 

 the method proposed by Professor Maxwell, and employed by Mr. 

 Gordon. The only vice in Mr. Gordon's employment of that method 

 was that plates of dielectrics of dimensions comparable with their 

 thickness were regarded as of infinite area, and thus an error of 

 unexpectedly great magnitude was introduced. 



For determining the capacity of liquids, the apparatus consisted of 

 a combination of four air condensers, with a fifth for containing the 

 liquid arranged as in a Wheatstone's bridge, fig. 1. Two, E, F, were 



Fig. 1. 



