360 Recent Advances in Telegraphy. [July, 
the vane B : thus a signal is made there. This corresponds 
to Station A signalling Station B. The other case is when 
both stations are sending to each other. In this case the 
water is simultaneously omitted at both ends of the pipe, 
and both vanes rotate. This corresponds to both stations 
signalling each other. Out of the distinct signals the 
message is of course made up. In duplex sending these two 
elementary cases are constantly recurring, owing to the 
pauses between letters and words. We have, for the sake 
of simplicity, considered that the sending current used at 
each station is of the same strength and of one kind, either 
positive or negative. But in submarine telegraphy both 
positive and negative currents are used in sending, so that a 
third case has to be added to the above — namely, when 
Station A is sending a positive and Station B a negative 
signal. A little reflection will, however, show that this is 
merely a variety of the second case. It will be seen, too, 
that the principle still holds good when the batteries used 
at the two stations are of unequal strength ; for the effedt 
will only change in degree. 
Immediately after the invention of the differential method 
there was'a great deal of activity displayed in duplex expe- 
riments, both at home and abroad, especially in Germany 
and America. The system was even in practical operation 
on many of the Prussian land-lines, but it gradually fell into 
disuse, chiefly through the difficulty in maintaining the 
equivalence of the real and artificial lines, owing to imper- 
fect insulation of the wires. Some desultory progress was 
still exhibited here and there, however, showing that the 
idea was only in abeyance until circumstances were more 
favourable to its development. The most noticeable of 
these improvements was the plan of putting the receiving 
instrument in the diagonal wire of a Wheatstone balance 
formed of the adtual line, the artificial line, and two other 
proportional resistances. This has been called the “ Wheat- 
stone Balance Method.” It originated with M. Maron, of 
Berlin, in 1863. The arrangement of apparatus at each 
station is shown in Fig. 4. The proportional resistances, 
r r' , are usually made equal to each other, so that by the 
principle of the Wheatstone balances the artificial line must 
be made equal to the adtual line. When this is so there 
will be no effedt on the receiving instrument placed in the 
diagonal cd when the sending key is depressed and the current 
from the battery admitted into the system. The current 
will divide itself at the point b, and one-half will flow by 
the branch r into the real line, and so on to the distant 
