102 ANNUAL OF SCIENTIFIC DISCOVERT. 



" The method of signalling: used between the ship and the land is 

 that now universally adopted in workin^]^ all long submarine lines 

 — thcretleeting galvanometer. The principle of this most deli- 

 cate iusiruniiMit was discovered a few years since by a (lermnn 

 electritian, named Weber. It was then, however, a large ma- 

 chine, and the condensation of all its powers into the smallest 

 and liglitest form is due to the scientific research and skill of Sir 

 Willi:im Thompson. 



" This instrument consists of a small mirror with a magneton its 

 back. That the two are very small ind(H'd may be judged by the 

 fact that both together weigh less than three-eighths of a grain. 

 This infinitesimally small retlector, which is intensely l)riglit, is 

 suspended by a silk thread, as line as a hair^ in the midst of a 

 small eircnlar coil of insulated copper wires. Directly a current 

 is sent through this circular coil, no matter how slight, it induces 

 another electric current within its circle, which acts in an opposite 

 direction, and this causes the magnet at the back of the mirror 

 to turn to the right or left, and, of course, to turn the little mirror 

 with its reflecting ray of light with it. By a veiy simple ar- 

 rangement, this line ray of light is thrown upon a horizontal 

 graduated scale, about 3 feet long and 3 feet distant from the 

 mirror. 



" Thus, when a current is sent through the little circular coil 

 around the mirror, the magnet is acted upon, and turns the mir- 

 ror with its ray of light, say on the left of the scale in front of it. 

 When the current is reversed, and that is instantly done b}' press- 

 ing a little key in the speaking instrument, the current in the 

 circular coil is reversed, and sent in the opposite direction, and 

 this in turn sends the ray of light from tlie mirror on to the 

 o})})osite side of the scale to the right. When the ray of light 

 rests stationary on any part of the scale, it means a dot; when it 

 moves rapidly to the right or left, it means so many dashes, ac- 

 cording to the distance it goes. This reflecting galvanometer 

 tells with unerring certainty whether or not the Great Eastern is 

 steady. 



'* The vessel now at the end of the cable is, with its coils of in- 

 sulated wire and iron hull, a mere electro-magnet, so to sj^eak. 

 The course of the Great Eastern is east and west, and therefore 

 at light angles with the course of the magnetic current, which is 

 north and south. Thus everj^ time the siiip rolls, either to port or 

 starboard, a slight current, but still a current, is induced in her vast 

 coils, and then transmitted through the cable to the shore end 

 at Minou, where it acts upon the reflecting galvanometer, and 

 turns, its rays of light a little to the right or left of the centre 

 of the scale, and thus shows in a fraction of a second of time 

 the precise degree and rapidity at which the vessel is rolling." 



THE FRENCH ATLANTIC TELEGRAPH. 



In the first place, it is interesting as being longer by about 

 1,500 miles, a|id laid in deeper water by oOO fathoms, than any 



