122 Mr. Snow Harris on Lightning Conductors 



14. Any continuous metallic rod or other body, m n (fig. 2.), 

 connected with the lower plane, must be considered merely 

 as a passive way of access for the charge so far as it goes ; 

 the electrical agency being observed to seize upon substances 

 best adapted and in a position to facilitate its progress, or 

 otherwise to fall with destructive effect upon such as resist it. 



15. It is easy to perceive here, that the presence of a con- 

 ducting rod, mn (fig. 2), or other conducting body, has no- 

 thing whatever to do with the great natural action set up be- 

 tween the planes A B. It is in fact to be considered merely 

 as a point in one of them. The original accumulation of elec- 

 tricity and subsequent discharge, would necessarily go on 

 whether such a rod were present or not, as is completely shown 

 by experience. When present, its operation is confined to the 

 transmission, so far as it extends, of that portion of the charge 

 which happens to fall upon it; and since it is quite impossible 

 to avoid the presence of conducting bodies in the construction 

 of ships, it is the more important to understand clearly in what 

 way damage by lightning occurs to the general mass, and how 

 it may be best avoided. 



16. When discharges of lightning fall upon a ship in the 

 way above stated, as being a heterogeneous mass fortuitously 

 placed between the charged surfaces A B (fig. 3.), the course 

 of the discharge is always determined through a certain line 

 or lines, which upon the whole least resist its progress. The 

 interposed air between the ship and the clouds first gives way 

 in some particular point, probably the weakest, — suppose at 

 A, fig. 3; — the electrical agency then meeting with continued 

 resistance from the non-conducting particles of air, is often 

 turned into a tortuous course. Suppose it arrives in this 

 way at some point, w, in the vicinity of a ship at k, the 



be accumulated on thick glass or on thin, the result is the same ; it is merely 

 the intensity as indicated by the electrometer which changes. 



Now the term free electricity, applies to the greater or lesser influence 

 of the opposed coating in respect of other bodies. In the case of the op- 

 posed surfaces of the clouds and earth, all the charge is necessarily free 

 electricity, since there exists no other point upon which it can tend to dis- 

 charge. In the same way the electricity of the jar, when the coatings are 

 very near, is nearly all redundant, or free electricity, in respect of the ac- 

 tion between them, although latent in respect of other bodies. Hence with 

 a moderate accumulation, the electrometer exhibits but a small intensity, if 

 any. The only diif>.rence at the time of the discharge, is in the position of 

 the discharging circuit, which in the case of the clouds and sea, is directly 

 in tlie interval of separation; and as we find the principle of induction al- 

 ways active in cases of lightning, the thickness of the stratum has evidently 

 no influence on the conditions of the accumulation, especially when we 

 consider the great extent of the opposed surfaces, which may possibly be 

 20,000 or more square acres. Dr. Faraday has shown that no distance 

 excludes the inductive action. 



