154 



NATURE 



[June 12, 1890 



buildings from the destructive effects of lightning. The 

 controversy originated in some lectures on lightning con- 

 ductors, delivered by Prof. Oliver Lodge at the Society 

 of Arts in 1888 ; it was continued at the Bath meeting of 

 the British Association, and it culminated in a paper, also 

 by Dr. Lodge, read last year at the Institution of Electri- 

 cal Engineers, in which, after stating that " the old views 

 on the subject of electrical conduction are hopelessly and 

 absurdly and dangerously inadequate," the author ex- 

 pressed the opinion that it was " time that the prophets 

 of the old superstition were slaughtered by the brook 

 Kishon." In the animated discussion which followed. 

 Dr. Lodge's views were ably opposed by Mr. Preece and 

 others, and the question can hardly yet be considered as 

 definitely settled. Time will not admit of an adequate 

 review of the arguments which were employed on the 

 two sides, but, considering its great practical importance, 

 I think it will be of interest to give a very short statement 

 of the matter in dispute, which I will illustrate by copies 

 of Dr. Lodge's diagrams and apparatus. 



Ever since the time of Franklin it has been customary 

 to make use of long pointed metallic rods for the purpose 

 of protecting important buildings from damage by light- 

 ning ; and the "older electricians," as Dr. Lodge calls 

 them, have always taught that, if the rod were well made, 

 of sufficient size and height, and properly connected to 

 earth, it afforded practically perfect security over a cer- 

 tain limited area. The function of the rod was supposed 

 to be not so much to receive the shock of a lightning 

 flash as to prevent a flash from occurring at all in the 

 neighbourhood of the protected building : this it did by 

 promoting the silent discharge of electricity between the 

 cloud and the earth through the point of the rod. 



The lower of these two tinfoil-covered boards repre- 

 sents the earth, and the upper one a cloud ; the upright 

 metal rod with a ball at the top of it is supposed to be a 

 church, or other building, erected upon the earth. Charg- 

 ing the apparatus by means of the electrical machine, we 

 get a series of strong flashes between the cloud and the 

 church, every one of which might do terrible damage. If 

 now we place near the church another rod, with a needle- 

 point at its end, to serve as a lightning-conductor, the 

 flashes at once cease : however vigorously we work the 

 machine, there is no longer any visible effect. The fact 

 is, that the electricity is silently and harmlessly discharged 

 as quickly as it is generated. In such a case as is at 

 present represented by the model, the efficacy of a light- 

 ning conductor would be complete. This is what Dr. 

 Lodge calls the case of " steady strain," and is that 

 indicated in his first diagram [exhibited], where the 

 charged cloud above the church spire is supposed to have 

 moved into its present threatening position from a dis- 

 tance. According to Dr. Lodge, this is the only kind of 

 lightning discharge which was ever contemplated by the 

 older electricians. 



But suppose that a harmless uncharged cloud which 

 might be hovering over the church were suddenly to re- 

 ceive an overflowing charge of electricity by a flash from 

 another more distant cloud. There would then be no 

 time for any gradual relief of the strain by a silent dis- 

 charge through the lightning conductor, and either the 

 conductor itself or the church would infallibly be struck 

 by a flash from the overflowing cloud. 



By altering the connections between the model and the 

 electrical machine,^ we can easily imitate this condition 

 of things. The tinfoil-covered boards now remain abso- 

 lutely uncharged until the moment when there is a spark 

 between the terminals of the machine : then they are 

 suddenly charged, and a flash instantly passes between 

 the cloud and the church. Placing the needle-pointed 

 lightning conductor beside the church, we now find that 



' The tinfoil-covered boards were ccnnected with the outer coatings of the 

 Leyden jars, their inner coatings being in connection with the terminals of 

 the machine. 



NO. 1076, VOL. 42] 



it is powerless to prevent the flashes : they go on just as 

 rapidly as before, striking either the conductor or the 

 church, or sometimes both at once. This case, which, I 

 think, Dr. Lodge was undoubtedly the first to call atten- 

 tion to explicitly in connection with thunderstorms, is 

 called by him the case of " impulsive rush." The occur- 

 rence of an "impulsive-rush" flash, then, cannot be 

 warded off by a lightning conductor. The most that a 

 conductor can do is to divert the main shock of the dis- 

 charge from the building to itself. But even so the 

 lightning may do considerable damage, for, as Dr. Lodge 

 says, " it is hopeless to pretend to be able to make the 

 lightning conductor so much the easiest path that all others 

 are protected. All possible paths will share the discharge 

 between them, and lots of apparently impossible ones." 

 Moreover, not only is the lightning conductor itself, when 

 struck, liable to spit off sparks laterally, however good 

 its earth connection may be, but other metallic bodies in 

 the neighbourhood may do the same, whether such bodies 

 are insulated or not. 



The moral appears to be this. In all cases of steady 

 strain in which a charged cloud descending from the 

 upper regions of the air, or approaching from a distance, 

 might inflict serious injury upon an unprotected building, 

 a well-designed and properly earth-connected lightning- 

 rod is an absolute safeguard. In a case of " impulsive 

 rush," the rod may often be of use in bearing the brunt of 

 the discharge, though sometimes the lightning will take 

 no notice whatever of it, striking the building and alto- 

 gether neglecting the rod ; and it is even possible that a 

 high rod might attract a destructive discharge which 

 otherwise would not have occurred at all. Although, 

 therefore, a lightning-rod is in many cases, probably in 

 a very large majority, of the greatest service, it cannot 

 be depended upon as affording perfect immunity from 

 risk; and the assumption which has universally been 

 made by the " older electricians," that damage by light- 

 ning is in itself conclusive evidence of some imperfection 

 in the conductor, is an unfounded one. 



In conclusion, it may not be out of place to say a word 

 or two on the subject of personal danger from lightning. 

 The spectacle of a severe thunderstorm, magnificent as it 

 is, is no doubt calculated to inspire a certain amount of 

 alarm. But statistics show clearly enough that, at least 

 in this country, its bark is worse than its bite. It appears, 

 from a paper published last year by Inspector- General 

 Lawson, that the number of deaths caused by lightning 

 in England and Wales from 1852 to 1880, as recorded in 

 the returns of the Registrar-General, were 546, or rather 

 less than 19 per annum. The average population during 

 that period may be taken as 22 millions ; it follows, there- 

 fore, that the average annual death-rate from lightning 

 was considerably below i per million of the population. 

 The risk of a fatal lightning stroke in any individual 

 case is therefore exceedingly small. 



SPORTSy 



IT is highly desirable that we should attach a definite 

 signification to this word. Among gardeners it may 

 mean many things, whilst, among botanists, it is restricted 

 to cases of bud-variation as distinguished from variation 

 from seed. In this note we shall use the word in its 

 botanical sense, as applying to a special illustration of 

 that tendency to vary which is common to all living 

 beings. We shall, however, gain a clearer idea of what 

 true sports are by the elimination of certain things which 

 are not sports, though often called so. In the first place 

 they are not seedling variations. Out of a hundred seeds 

 of Lawson's Cypress that are sown it is possible, I suppose, 



I Reprinted from Garden and Forest. The article contains the substance 

 of an unwr.tten address lately given to a society of gardeners. 



