1840.] On Lightning Conductors to Powder Magazines. 287 



wards, but the part that is under ground I should turn from the 

 building in its course through the earth, and take especial care, 

 by plates of copper, to make its contact with the moist earth 

 extensive and good. 



Conductors should be of a certain height in relation to the 

 roof or summit of the building to be defended ; a lightning rod 

 rising ten feet above any part of the roof or chimneys of a 

 house, might defend that house perfectly if close to it, but not 

 if ten feet from it ; a rod rising fifteen feet above the highest 

 parts of the roof would be more sure than one of ten feet. 



A rod projecting ten feet which would protect a building of a 

 certain horizontal extent might protect a building ten feet wide, 

 &c. A lightning rod has been considered as able to protect 

 objects perfectly when they are not more than twice the distance 

 from it of its height above them : but for this to hold true, these 

 objects should not be themselves parts of large masses of metal, 

 approaching by their position or connexion to the character of 

 bad lightning conductors. 



I have no fear of lateral discharge from a well arranged con- 

 ductor. As far as I understand lateral discharge, it is always 

 a discharge from the conductor itself; it might be very serious 

 from a badly arranged conductor (and in fact makes them worse 

 than nothing) but with a good lightning rod it can be but small, 

 and then not to badly conducting matter, as wood or stone, but 

 only to neighbouring masses of good conducting matter, as 

 the metals, which either ought not to be there, or if they are 

 necessarily present, ought to be in metallic communication with 

 the lightning conductor itself. I am not aware that lateral dis- 

 charge can take place within a building when a lightning con- 

 ductor outside is struck, except there be portions of metal, as bell 

 wires, or bolts, &c. which may form an interrupted conducting 

 train from the conductor to the interior.* It is true that cases 

 which come under the denomination of returning stroke, might 

 perhaps produce a spark in the interior of a building, but the 

 phenomena of a returning stroke cannot occur at the place 

 where the lightning strikes a conductor. 



* Such as the copper linings of powder barrels in a magazine — W. B. O'S. 



