HISTORY AND PROGRESS. 211 



the Prussian state telegraph. It consists of a double cone, 

 supported by a stout metal bar, in connection with the earth. 

 The points are in close proximity with the points of two metal 

 cones, which are supported on a common base, and severally 

 connected with the lines, and also with the two ends of the 

 wire coils of the receiving apparatus. When the line con- 

 tains free static electricity, the latter springs from the cone 

 on the side on which the line is struck to the little double 

 cone in the middle, and thus avoids the apparatus. 



111. Siemens and Halske's Point Lightning Discharger. 

 A lightning- guard of beautiful construction, for protecting 

 submarine wires which are connected with overland lines, is 

 used by Messrs. Siemens and Halske, which, while sufficiently 

 serving the same purpose as those already described, has the 

 advantage of being fixed upon the instrument board, where 

 no difficulty can arise in mounting it, and any disorder in 

 which it may get will be more readily discovered. It is 

 formed of a cube of brass connected with the earth, to three 

 sides of which are presented sharp points of metal protruding 

 from an arch in the circuit of the line wire. The brass prism 

 is faced with three metal plates, carrying agate cones on the 

 top and on each side under the arch. The purpose of the agate 

 cones is to prevent the points being adjusted too close to the 

 earth-block. The points are formed by three screws, the axes 

 of which lie in the same plane, one running vertically through 

 the top of the arch and the other two horizontally on opposite 

 sides. Within each of the screws, along its axis, is a second 

 screw of small diameter, terminating in a conical point of 

 platinum. 



112. Breguet's Lightning Discharger. Breguet has con- 

 structed another lightning discharger, now used on all the 

 French lines, in which he increases the means of discharging 

 the static electricity from the line by increasing the number 

 of points over which it can pass, and diminishing the resist- 

 ance to its passage. For this purpose he arranges two plates 

 of copper insulated from each other upon a common board. 

 The opposite edges are cut out in the form of sharp teeth, 

 so that the point of each tooth on one plate is opposite the 



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