PROTECTION FROM LIGHTNING. 461 



flashes, which were perhaps side branches or minor spitting-off 

 discharges. 



The risk, then, will vary with the discharge, and this is influ- 

 enced somewhat by locality, and therefore the methods of protec- 

 tion to be employed in the Mississippi Valley would be somewhat 

 different in character from the methods appropriate for, say, the 

 New England coast ; the frequency of thunderstorms in the one 

 place compared with the other being about four to one. The 

 character of the storm is also somewhat different. Then again the 

 liability for places comparatively near is not the same. " If I 

 urge on Glasgow manufacturers," said Sir William Thomson, " to 

 put up lightning conductors, they say that it is cheaper to in- 

 sure." These manufacturers answered the man of science more 

 wisely perhaps than they themselves knew. Thanks to the inves- 

 tigations of the Prussian Bureau of Statistics, we know now that 

 in the main in thickly settled communities the risk is small. We 

 can state with some certainty that there is but little need for the 

 erection of expensive or elaborate lightning rods upon buildings 

 standing among others in city blocks. We do not say that such 

 buildings are never struck. As we have seen above, under some 

 conditions, "exposure" seems to have little to do with the 

 determination of the path of discharge. The case is somewhat 

 analogous to that of the trees and rocks upon a mountain side. 

 However much they may determine the course of small streams 

 and " trickles " down the mountain side, they are powerless to in- 

 fluence the course of an avalanche or land-slide. Sometimes, there- 

 fore, such buildings are struck and severely injured (and as might 

 be expected, with seemingly good protectors, do not entirely es- 

 cape) ; but these cases are rare, and it may, we think, be safely 

 set down that rods upon city houses are not (as hitherto insisted 

 upon by some) necessary. With country houses the conditions 

 .are different. 



Our next question is, In flashes of ordinary intensity how 

 much confidence may be placed in the protection afforded by a 

 good conductor, rod or tape ? Few questions have been so thor- 

 oughly discussed from a practical standpoint, and the verdict 

 may be given in Sir William Thomson's words : " There is a 

 very comfortable degree of security, if not of absolute safety, 

 .given to us by lightning conductors made according to ortho- 

 dox rules." 



If the reader is contemplating the erection of a lightning pro- 

 tector, these points may be of service to him : 



1. Get a good iron or copper conductor of rod or tape form, 

 preferably the latter. If copper, have it weigh about six ounces 

 "to the foot ; if iron, about two pounds to the foot. 



2. The nature of the locality will determine to a great degree 



