550 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



on two of the buttons marked with the figures 7, 8, 9, 10, 11, 12, 

 13, 14, 15, 16, 17, and 18, placed in two different horizontal rows, 

 the current passes through the two corresponding galvanometers after 

 having traversed the line wires and put in action the same needles 

 of the receiving dial. 



We will not describe the mechanism of the manipulator of this 

 system, though it was successfully worked on the London and Birming- 

 ham Kail way, until replaced by a simpler system ; in fact, Wheatstone in 



FIG. 350. Cooke and Wlieatstoiie's single needle telegraph manipulator and indicator. 



conjunction with Cooke soon modified it, by reducing the number of 

 galvanometers to two or even one. Hence arose the single needle, 

 and two needle telegraphs which have been adopted on English tele- 

 graphic lines, and which we will now describe. The mechanism is 

 as we shall see, of great simplicity. 



Fig. 350 represents, on the left, the front face of the apparatus, 

 which is the same at the receiving and sending stations. 



In the centre, we see the outer needle of the galvanometer whose 



