62 Louis Schwencller — On the General [No. 2, 



Therefore b becomes largest for any given c and any given (« + d), if 

 we put a = d. 



Bat b largest is required for two separate reasons : 



1. If the immediate balance is disturbed by an alteration of the resis- 

 tance of one or more of the four branches, which may happen, especially by 



f, i. e., (3 (battery resistance) varying, then p becomes at once a function of 

 b, i e., an increasing one with b. Thus in order to keep p as large as pos- 

 sible, and at the same time as constant as possible, b should be selected 

 largest. 



2. Further by making b as large as the circumstances will admit, we 

 clearly have the largest sent and largest received currents, which will be 

 clear without calculation. In fact later on, page 232, it has been shewn that 

 a = d is the condition for the maximum signalling current. 



' Addendum II. 



Since the 3rd February, 1875, the main line from Bombay to Madras 

 had been successfully worked duplice by means of the " double balance 

 method." 



This line is worked direct, i. e., without any translating instruments, 

 and is 797 miles in length ; it consists almost throughout of No. 5|- wire 

 B. W. Gr. (diameter 5^ m. m.) and is supported chiefly on the Prussian 

 insulator. 



The section of this line from Bombay to Callian is exposed to the de- 

 structive influence of a tropical sea climate ; between Callian and Poona the 

 line passes over the Western Ghats, the dense fogs during the cold weathe r 

 and the heavy rains during the South-west monsoon on these hills seriously 

 affect its insulation ; from Poona to Sholapore and Bellary, the line runs in- 

 land and experiences a climate on the whole favourable for the maintenance 

 of constant and high insulation ; between Bellary and Madras, however, the 

 line again comes under the influence of a most unfavourable climate, especial- 

 ly just before and during the continuation of the North-east monsoon, when 

 the atmosphere at a high temperature, is saturated with moisture and salt, 

 leaving conducting deposits on the surface of the insulators. 



Consequently during the South-west monsoon the resultant fault is 

 near Bombay, during the hot weather it shifts towards the middle of the 

 line, and in November when the rains set in at Madras and the weather on 

 the Bombay side is clearing up, the resultant fault is situated close to 

 Madras. 



