132 



♦ KNOWLEDGE ♦ 



[Aug. 1.3. 1884. 



irregular surface, upon which is accumulated all the elec- 

 tricity that was previously distributed over the surfaces of 

 the miilions of particles that now compose it. 



The tendency of an electric charge upon the surface of 

 a conductor is to take upon itself a position in which it 

 may approach nearest to an equal and opposite charge, or, 

 if possible, to attain neutrality. If, then, a cloud has a 

 charge, and there is no other cloud above or near it, the 

 charge induces on the adjacent earth surface, electricity 

 of the opposite kind. Thus, assuming the cloud to be 

 charged with positive electricity, the subjacent earth will 

 be in the negative state. The two electricities* exert a 

 strong tendency to combine or to produce neutrality, whence 

 there is a species of stress applied to the intervening air. 

 Possibly the cloud will be drawn bodily towards the earth 

 more or less rapidly, according as the charge is great or 

 .small. Or, on the other hand, the cloud may roll on for 

 leagues, carrying its influence with it, so that the various 

 portions of the earth underneath become successively charged 

 and discharged as the cloud progresses on its journey. 



Should the cloud be near the earth, or should it be very 

 highly charged, the tension of the two electricities may be 

 80 great as to overcome the resistance of the intervening 

 air ; and if this resistance should prove too weak, what 

 happens f How does the discharge show itself 1 It takes 

 place in the form of a lightning flash, and passing from the 

 one surface to the other — or, may be, simultaneously from 

 both — produces neutrality more or less complete. 



There has recently been a little discussion in these pages 

 on the subject of lightning, some having stated that they 

 discerned the discharge to take place upwards — that is, 

 fi-om the earth towards the cloud. I will not venture so 

 far as to say whether or not the direction of the discharge 

 is discernible ; possibly the flash may sometimes be long 

 enough to enable one to tell ; but I have never so seen it, 

 and have always looked upon the eye as a deceitful member 

 — very. " The lightning flash itself never lasts more than 

 To oWo °^ * second." It is, however, just as likely that a 

 discharge may travel upwards as downwards. What con- 

 trols the discharge? Does the quality of the charge? — 

 that is to say, is the positive or the negative move prone to 

 break disruptively through the insulating medium ? Investi- 

 gations with Geissler's and other tubes containing highly 

 rarefied gases have made it tolerably clear that there is a 

 greater " tearing away " influence at the negative than at 

 the positive pole, and if two equal balls, containing one a 

 positive and the other a negative charge, be equally heated, 

 the negative is more readily dissipated than the positive. 

 But, so far as we at present know, this question enters 

 into the discussion scarcely, if at all. Our knowledge 

 seems rather to point to the substances upon which the 

 charges are collected. The self-repellent nature of elec- 

 tricity compels it to manifest itself at the more prominent 

 parts of the surface, the level being forsaken for the point. 

 The tension of the charge, or its tendency to fly-off, is 

 proportionately increased. And if at a given moment the 

 tension attains a certain intensity, the discharge follows, 

 emanating from the surface which offers the greatest 

 facilities for escape. The earth is generally flatter than 

 the cloud, whence, in all probability, the discharge more 

 frequently originates with the cloud. 



Should a lightning flash strike the earth and produce 

 direct neutrality, it is possible that no damage will result, 



* We may speak of two electricities or two electric states without 

 neceBsarilv implying adherence either to the single or the double 

 " fluid " theory. Whether electricity be of two kinds or no, the 

 fact remains that there are two conditions, and all the features of 

 this paper may be explained with equal facility by the supporters 

 of either hypothesis. 



although this again is not always certain, because when the 

 cloud-charge acts inductively upon the earth it produces 

 the opposite (say negative) charge on the nearer parts, the 

 similar (or positive) state is also produced at some place 

 more or Itss distant. Sometimes this " freed " positive 

 (which, by the way, accumulates gradually and physiologi- 

 cally imperceptibly) is collected at some portion of the 

 earth's surface. When the negative is neutralised by the 

 discharge, the freed positive is no longer confined to a par- 

 ticular region, but tends to dissipate itself, and a shock may 

 be felt more or less severely by any person within the 

 region. Or, again, a similar shock may be experienced by 

 a person standing within the negative zone on the neutrali- 

 sation of the charge. 



I may take the opportunity here to mention a highly 

 interesting and instructive incident observed on local tele- 

 graph circuits during a thunderstorm. The storm may be 

 taking place at some distance from the point of observa- 

 tion. The electrified cloud induces the opposite charge 

 beneath it, the similar charge being repelled. It is 

 noticeable that the needle of a galvanometer, starting from 

 the middle position, goes gradually over to one side, 

 eventually indicating a considerable deflection. Suddenly, 

 owing apparently to a lightning discharge some distance 

 away, the force which caused the deflection is withdrawn, 

 and the needle rebounds with great violence to the opposite 

 side. In a short time, the cloud becoming again charged on 

 its under surface, and recommencing its inductive eflfect 

 upon the subjacent earth, the needle starts again, and goes 

 through the same series of movements, a violent counter- 

 throw following every flash of lightning. 



If we can so far control our imagination, we may con- 

 ceive the earth to be one large insulated conductor, suscep- 

 tible to every influence around it If, then, the earth, 

 as a mass of matter, behaves as above indicaf;ed, there is no 

 plausible reason for declining to regard any other large 

 conducting mass in a similar Ught, and, as a body capable 

 of being subjected more or less completely to the various 

 impulses affecting the earth. In other words, a large mass 

 of conducting material, partially or perfectly insulated is, 

 during a thunderstorm, in considerable danger. With 

 this portion of the subject I shall, however, deal more fuUy 

 when discussing the merits of lightning protectors. 



Lightning discharges do not take place between 

 cloud and earth only, but also, and perhaps more 

 frequently, between two oppositely-charged clouds. We 

 then get atmospheric lightning, the flash often extending 

 for miles. This form of lightning is harmless, and in all 

 probability what we see is only a reflection of the dis- 

 charge. The oft-told tale of the lightning flying in at the 

 window, across the room, and out of the door, or up the 

 chimney, is all moonshine, and before dealing with lightning- 

 protectors I intend to expose some of the fallacies con- 

 cerning lightning. Were the discharge to pass through a 

 house it would infallibly leave more decided traces and do 

 more damage than simply scaring a superstitious old lady 

 now and again. Many people are often and unnecessarily 

 frightened during a thunder-storm, but it may be safely 

 predicted that a person under a roof is infinitely safer than 

 one who is standing alone on level ground, and making 

 himself a prominence inviting a discharge. Rain almost 

 invariably accompanies the discharge, and the roof and 

 sides of the house being wet, they form a more or less 

 perfect channel of escape should a flash strike the building. 



Glass-making ix Italy. — Colle de Val d' Elsa contains one of the 

 most important glass-works in Italy. The value of the annual pro- 

 daction amounts to 2,500,000 lire; its sale extends overall Italy, but 

 more especially in the Boman, Tuscan, and Piedmontese provinces 



