ABSTRACTS OF SCIENTIFIC PAPERS 357 



of some submarine cable, out of sheets of tinfoil separated by 

 paraffined paper, gutta-percha, or mica. Mr. Jenkin adopted Mr. 

 Latimer Clark's construction, in which mica formed the insulator, 

 and made a condenser whose capacity he adjusted so that it should 

 be approximately equal to 10~ 14 abso'ute electromagnetic metre- 

 gramme-second units. He points out that a tenfold multiple of this 

 standard [or what is now known as a Microfarad] would be a con- 

 venient unit in submarine telegraph work. The capacity of the 

 condenser was measured by comparing the discharge from it through 

 a ballistic mirror galvanometer, whose needle was weighted for the 

 purpose, with the steady deflection produced when the battery used 

 to charge the condenser was placed in circuit with the galvanometer 

 through a known large resistance. Reference is made to the absorption 

 and residual discharge phenomena exhibited by mica in common with 

 other solid dielectrics, and to their influence in causing the capacity 

 of a mica condenser to be somewhat indefinite. The experiments 

 are recorded in full. 



XVII. On the Retardation of Electrical Signal* on Land Lines. 

 ' British Association Report for 1864 '; ' Philosophical Magazine,' 

 June, 1865. 



The purpose of this paper is to examine the application of Sir 

 W. Thomson's theory of the transmission of electric signals to land 

 telegraphs, in the light of certain experiments published by M. 

 Guillemin in 1860. The author gives a short statement of the results 

 of Thomson's theory, and quotes, on the authority of Mr. Charles 

 Hockin, the following series as a convenient expression to be used 

 in drawing the ' curve of arrival : ' 



Where 



x is the current at the receiving end, after any interval t from 



the time of application of the battery. 

 C is the current at the receiving end after an indefinitely long 



application of the battery. 



k being the resistance of the conductor, per unit of length ; c the 

 capacity per unit of length, both in absolute electrostatic measures 



