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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



this building, and through it to and from the 

 ground. The effects of the passage of electricity 

 through the building will be determined by the 

 degree of conductivity possessed by the latter, 

 or, in other words, by the resistance it offers. 

 If it be constructed of metal, or if it have a con- 

 tinuous piece of metal running through it from top 

 to bottom, the electric charge will pass without 

 causing in it any visible effect, because little or no 

 resistance is opposed. For this reason buildings, 

 as the reader doubtless knows, have often affixed 

 to them for their protection a metal rod called a 

 lightning-conductor. But if there be no metal 

 employed in the construction of the building, or 

 if the metal used be in separate and detached 

 pieces, there will be great resistance to be over- 

 come, and the force required to overcome it may 

 be sufficient to cause the destruction of the build- 

 ing, or at least the displacement of those parts 

 where the resistance is greatest. The heat gen- 

 erated by the passage of electricity through badly- 

 conducting substances is often great enough to 

 set combustible materials on fire. 



Instead of a building, the object causing the 

 diminution of resistance may be a tree. The 

 moisture contained in a living tree renders it a 

 moderately good conductor of electricity, and on 

 that account it is more likely to attract lightning 

 than an ordinary stone building. So also the 

 human body, which is a better conductor than a 

 tree, may, by occupying a favorable situation, be- 

 come part of the line of least resistance ; in such 

 a case, the passage of the electric charge will take 

 place through it, as it did through the tree and 

 the building. But since both the tree and the 

 human or any other animal body possess a low 

 degree of conductivity, the consequent force ex- 

 erted may cause their destruction. Thus we see 

 that a body is " struck " when it becomes part 

 of a line of least resistance ; and, from a consid- 

 eration of the foregoing facts, we are able to per- 

 ceive when a body is likely to become part of 

 that line. We also see that good conductors, 

 such as the metals, are unaltered by the passage 

 of the charge through them ; while imperfect 

 conductors, such as trees and animal bodies, are 

 either injured or destroyed by it. The electric 

 fluid revenges itself, as it were, upon whatever 

 offers resistance to its course. 



We may now endeavor to ascertain under 

 what conditions damage from lightning is pos- 

 sible, and what are the means by which the risk 

 may be lessened or avoided. An isolated tree, 

 standing either upon a wide plain or upon an em- 

 inence, is obviously likely to determine a light- 



ning-discharge, to " attract the lightning," to use 

 a common expression. The top of the tree is the 

 nearest point to the cloud ; and since the tree is 

 a better conductor than the air, a line drawn ver- 

 tically through it to the cloud marks the shortest 

 and easiest course along which the electricities 

 may pass. If, when the charged cloud arrives 

 directly over this point, the tension is sufficient 

 to overcome the resistance along that line, a dis- 

 charge will take place, and the tree will be struck. 

 But if the tension be not sufficient, the cloud will 

 pass harmlessly over. Hence it appears that a 

 person standing during a thunder-storm beneath 

 a tree so situate is exposed to some risk. On 

 no account, therefore, should the traveler take 

 refuge under an isolated tree ; generally, he will 

 do well to avoid its neighborhood altogether ; 

 but should he be overtaken by the storm when 

 on a plain with no shelter near, the tree may still 

 be made to afford him some protection. If he 

 take up a position near it, but not under its 

 branches, he will probably escape unhurt should 

 the lightning descend upon it. The safest dis- 

 tance from the tree is that which is equal to its 

 height. To approach much nearer than this is to 

 incur the risk of being within the influence of 

 the stroke ; to remain at a much greater distance 

 away is to place one's self in the same condition 

 of isolation as the tree itself. It will have been 

 remarked by all observers of the phenomenon, 

 that whenever a tree has been struck by light- 

 ning it has generally occupied an isolated posi- 

 tion. In describing the position as one of isola- 

 tion, however, it is not meant that the tree is 

 necessarily standing alone, but that it is not one 

 of a numerous group. When there are many 

 trees together, their collective conductivity is 

 often sufficient to cause an indestructive dis- 

 charge of the electricity. This is especially like- 

 ly to happen when the trees are wet with rain, 

 for then their surfaces are covered with a film of 

 water, which is a good conductor. For this rea- 

 son, the danger from lightning is much less after 

 rain has begun to fall, than before when every- 

 thing is dry. 



It appears, therefore, that the safest situation 

 during a thunder-storm is in the midst of a wood, 

 particularly if the neighborhood of the tallest 

 trees be avoided. In such a place of shelter, the 

 traveler may, take refuge in full assurance that 

 he will there be effectually shielded from harm. 

 The greatest risk of injury from lightning is un- 

 doubtedly incurred by persons traveling across a 

 wide and very flat plain, because in such a situa- 

 tion they are the only elevated objects. To lessen 



