1 877.] 
359 
Recent Advances in Telegraphy . 
water analogy. Let us suppose that Fig. 4 is a peculiarly- 
shaped water-pipe. The spoked wheels or vanes in the 
enlarged parts of the pipe are free to rotate, and represent 
the needles or indicators of the receiving instruments at the 
Stations A and B. The long conneCting-pipe, L, represents 
the line between these stations, and the short pipes, a l, 
stand for the artificial lines. Confining our attention to 
Station A, if we suppose that water at a certain pressure is 
admitted into the pipe at a, it will divide itself into two 
streams, one of which will flow over the vane into the pipe 
l, and the other will flow under the vane into the pipe A l, 
as shown by the curved arrows. If the pipes are so con- 
structed that these two streams are equal in force the vane 
will remain stationary, for they will balance each otheBs 
Wheatstone Bridge Method. 
O lTTi£/ 
effedt on it. But if by some means the stream flowing 
along the pipe l should be stopped, the balance on the vane 
would be disturbed, and it would be seen to rotate. Such 
an effeCt would be produced by admitting a like stream of 
water into the pipe by the orifice b at the other end. When 
the water is simultaneously admitted into the pipe at both 
ends the two opposing streams in the pipe l stop each other, 
and simultaneously both vanes move. Thus, when both 
stations admit water, both vanes move. We thus arrive at 
the two elementary cases of duplex working. The first case 
is when one station (say A) is sending a signal, while the 
other station is not. We have seen that in this case the 
water is only admitted at A, and the vane there is mo- 
tionless ; but the stream which flows along the pipe L rotates 
