ELECTRICAL SCIEXCE. 163 



The inventors of this improved target are Messrs. John Lang 

 and Charles Chevalier, of the Submarine Telegraph Works, Birken- 

 head. The object of their invention is to enable a marksman to 

 ascertain instantaneously, by means of self-acting electrical appa- 

 ratus, the effect of his shot upon a target, — that is to say, to indi- 

 cate upon the spot from whence a bullet is fired, or at any other 

 convenient place, what part of the target has been struck by the 

 bullet without the necessity for a marker near the target. In order 

 to effect this, the target is made of several strong plates suspended 

 from a suitably strong framing, and behind each plate are fixed in- 

 sulated metallic points. Each plate or segment of which the target 

 is composed is connected with one pole of a galvanic battery, and 

 the points behind such plates or segments with the other pole, and at 

 any convenient point of the circuit is placed a needle surrounded 

 with a coil in such a manner that when any of the plates or seg- 

 ments of which the target is composed is struck by a bullet, it is 

 caused to move backwards till it comes in contact with the points 

 behind it, and thus forms electrical connexion, which, completing 

 the circuit, causes the needle to be deflected. Each of the plates or 

 segments being furnished with its special circuit and indicating 

 needle, by inspecting these indicators is demonstrated which plate 

 has been struck by the bullet, as only the needle belonging to that 

 particular plate is deflected. But, instead of connecting each piece 

 or portion of the target through a separate conducting wire with a 

 separate needle or indicator, it is preferable for most purposes to 

 reduce the number of conducting wires, .and dials, and needles, and 

 rely upon the changes and permutations which can be made or 

 effected upon one dial with one needle, consistent with fewness of 

 parts, simplicity of action, and the proper recording of the vibrations 

 of the needle or pointer. — Mechanics' Magazine. 



GUTTA-PERCHA AND IXDIA-RUBBER. 



A paper has been read to the British Association, " On the Cha- 

 racter and Comparative Value of Gutta-Percha and India-rubber 

 employed as Insulators for Subaqueous Telegraph Wires," by Mr. S. 

 W. Silver. After pointing out some of the mistakes prevalent on 

 the subject of the insulating properties of India-rubber, a comparison 

 was made by the writer between the relative advantages and the 

 insulating power of India-rubber and gutta-percha respectively. 

 Insulation in the case of a submarine cable depends upon two causes 

 or properties of the bodies used : — 1. The specific non-conducting 

 power of the substance ; 2. Its impermeability, by which the original 

 insulating conditions may be maintained. The insulating power of 

 gutta-percha is very hi«h ; but, in the case of a submarine telegraph 

 cable, its porosity renders it a very imperfect insulator in practice- 

 India-rubber, with lower specific insulating properties (as would appear 

 from experiments made in dry air), is, nevertheless, practically a far 

 more efficient insulator, by reason of its complete impermeability, 

 while in addition it possesses a lower inductive capacity. It was 

 pointed out that impermeability is as important a question as specific 

 L 2 



