866 



SCIENCE 



[N. S. Vol. XXX. No. 781 



hnndred feet or more from the ground, by 

 far the major part of the air layer has been 

 so weakened electrically by the presence of 

 the water drops, that the discharge itself 

 anticipates the completion of the distance 

 of fall of the rain, and is therefore a short 

 time in advance of the time when the de- 

 scending gush of rain actually reaches the 

 ground. A& the gusts or gushes of rain 

 are more or less local and sweep along with 

 the storm cloud, they are apt to mark out 

 the places of the most frequent lightning 

 strokes. Shelter sought at such times 

 under tall trees is particularly dangerous. 



The amount of energy which may be 

 concerned in a lightning discharge is 

 neither definite nor capable of estimation. 

 It would seem that the widest variations in 

 energy may occur and this would account 

 largely for the observed diiierences in the 

 severity of the effects. It must be remem- 

 bered also that by far the larger part is 

 expended in the long spark in the air and 

 cloud. Even when much damage is done 

 to objects struck it is only a small fraction 

 of the total energy which is expended on 

 them. Most of the damage to property 

 comes indirectly from the electric discharge 

 by its energy being instantaneously con- 

 verted into heat. This heat evolves steam 

 and expanded gases in the interior of such 

 materials as wood and causes explosion, 

 shown in the splintering or rupture. 



A ciirious effect, often noted when a tree 

 is struck and shattered, is that when the 

 splinters, sometimes of large size, are 

 thrown bodily out to distances of many 

 feet from the shattered tree, the splinters 

 in their movement remain in parallel to the 

 tree and in a vertical position. They are 

 frequently found standing upright after a 

 stroke and at distances ranging up to sixty 

 or eighty feet away. This fact indicates 

 that the projecting force is quite instan- 

 taneous and is exerted equally and at the 



same moment throughout the length of the 

 splinter in a direction transverse to its 

 length. Such splinters are sometimes ten 

 or twelve feet in length and several inches 

 thick. As, will be seen, a person near a 

 large tree which is so disrupted is in danger 

 of being struck in a different way, even if 

 he escapes being included in the path of the 

 stroke itself. Aside from this mechanical 

 danger it is known that to take refuge 

 under a tall tree during a heavy thunder- 

 storm is particularly hazardous. This is 

 so because the human body is a better con- 

 ductor than the tree trunk, particularly as 

 the ti'unk itself is the last part to become 

 thoroughly wetted by the rain. The leaves 

 and upper parts are wet and more or less 

 conducting while the tree trunk itself may 

 be yet dry. In such a case the body of a 

 person forms a good path or shunt to the 

 dry trunk and is therefore particularly apt 

 to be traversed by any stroke which reaches 

 the tree. 



As before indicated, damage to buildings 

 and other such structures can in all cases 

 be prevented by the provision of an effect- 

 ive shunting path to earth. A most essen- 

 tial feature of such a structure as the 

 Franklin conductor is its good connection 

 with the ground, or better its connection 

 with what we know as a good gi'ound. In 

 early times it was considered that it was 

 quite important that the tip or upper end 

 of the conducting rod should be sharply 

 pointed, or should bristle with sharp points, 

 so to speak. The tips were gilded and the 

 points made of gold or platinum to prevent 

 rusting. The points were supposed to draw 

 off the lightning silently from the cloud 

 and so prevent strokes of lightning. But 

 for millions of volts at cloud distances 

 almost all irregular objects on the surface 

 of the earth are practically pointed. Per- 

 haps on this erroneous assumption of the 

 action of points as applied here little stress 



