G18 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



are kept in check by a tongue fixed in the lower part ; c is the 

 commutator of the apparatus ; B the manipulator with two keys, 

 analogous to Morse's manipulator, making positive and negative 

 currents alternately. To the negative currents correspond the 

 deflections of the needle and mirror to the left, to the positive 

 currents those to the right. F is a darkened chamber enclosing the 

 scale on which are formed the images of the name of the lamp situated 

 behind it. The luminous beam, passing through a hole in the side of 



ttf lf.lt 7i 



FIG. 404. Transatlantic telegraph from Brest to St. Peter's general view of Thomson's receivin 



apparatus. 



the chamber, follows the path H, falls upon the mirror, and is reflected 

 to the zero of the divided scale when the mirror is unmoved. At 

 each passage of the current sent through the cable, the mirror 

 oscillates to the right or left, as we have just seen, and the image 

 oscillates horizontally on one side or the other of zero. At A is a 

 battery of twenty Daniell's elements ; at j, the communication is made 

 with the earth. 



The clerk at the station, when he has received warninf that 



