67 



graphic communication, in a political and commercial' 

 light, the lecturer introduced some very delicate receiving 

 apparatus, to show the great difference between ocean 

 and land telegraphy. He explained that instead of the 

 noise, glare of light, clicking and bustle of an ordinary 

 telegraph office, the cable office was silent and dark, to 

 enable the watchful operator to detect the slight deflection 

 of the tiny pencil of light which was to impart to him 

 the intelligence which had flashed along under a thousand 

 watery leagues from a distant part of the world. After 

 explaining the reversing key and the different methods of 

 working cables, by reversal and change of potential, the 

 lecturer explained by diagrams some of the methods of 

 locating a fault or a break a hundred or a thousand miles 

 away from the shore, down deep upon the bottom of the 

 ocean ; also how a steamer would go almost directly over 

 the spot, find, take up and repair the fault. Several 

 specimens of cables were exhibited. 



A very sensitive tangent galvanometer with a small 

 reflector within its coils was arranged to receive a small 

 ray of light coming from a lamp, through a small hole in 

 the side of a box. On the back of the reflector was fas- 

 tened a very small magnetic needle, which was deflected 

 to the right or left in obedience to the positive or negative 

 current sent through the wire by the reversing key at the 

 sending station. The ray of light from the darkened box 

 falling upon the reflector through a convex lens, was re- 

 flected upon a screen at the top of the box. 



A positive current through the galvanometer would 

 throw the little spot of light to the right of zero on the 

 screen. A negative current would throw it to the left, 

 thus producing the combination, which to the practised 

 eye formed the letters of the alphabet. 



The lecturer then gave an illustration of the very deli- 

 cate and difficult process of finding a break or fault in the 



