388 WRITINGS OF JOSEPH HENRY. [1855- 



metal formed part of the building or the roof. Observations 

 have been recorded of parts of houses being struck within 

 the limit just mentioned as that of protection; but scarcely 

 any of them are satisfactory in determining the point, since 

 it appears from the evidence that in several cases there 

 were separate masses of metal which formed independent 

 conductors, and in the other cases there was no evidence 

 that the rod was in proper connection with the earth. In 

 order to protect an extensive building, it will evidently be 

 necessary to arm it with several lightning-conductors, and 

 the less their height, the greater must be their number. 



In the case of a tall steeple, it may be well to establish 

 points at different elevations, by branches from the main 

 rod ; for if it be true that the rod merely attracts the light- 

 ning which has been determined by the earth itself, or some 

 material under the ground, the discharge in its passage along 

 the line of least resistance to the point at which it was aimed, 

 may not be made to deviate from its direct course by the at- 

 traction of the distant elevated point, and may strike a lower 

 portion of the building. Suppose for example a thunder- 

 cloud is on the west side of a high steeple, and the point of 

 attraction, which may be damp earth, a pool of water, or 

 other conducting material on the surface or under the ground 

 at the east end of the church : the discharge from the cloud, 

 in its passage to the point of attraction, may strike a lower 

 portion of the building, the action of the elevated point not 

 being sufficient to deflect it from its course. This inference 

 is in accordance with actual observation. Mr. Alexander 

 Small wrote to Franklin, from London, in 1764, that he had 

 seen in front of his window a very vivid and slender light- 

 ning discharge pass low down, without a zig-zag appear- 

 ance, and strike a steeple below its summit. 



It becomes a matter of interest to ascertain whether the 

 action of an assemblage of conductors, such as is usually 

 found in cities, produces any sensible effect in diminishing 

 the electrical intensity of the cloud, or in other words whether 

 their united influence produces any sensible diminution of 

 the destructive effects of thunder-storms. Late researches 



