342 



NA TURE 



\_Feb. lo, 1 1 



LETTERS TO THE EDITOR \ 



! 



\The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake to 

 return, or to correspond tvith the writers of, rejected manu- 

 scripts. No notice is taken of anonymous communications. 



[ The Editor urgently requests correspondents to keep their letters 

 as short as possible. The pressure on his space is so great 

 that it is impossible otherwise to insure the appearance even 

 of communications containing interesting and novel facts.'\ 



Lightning- Flashes 

 The brief note (Nature, vol. xxxv. p. 85) giving the results 

 of the observations of Herr Leyst, of the Pawlowslc Observa- 

 tory, on the anomalous forms of lightning- flashes, suggests 

 several considerations relating to this class of phenomena. 



Leaving out of view the exceptional and anomalous phenomena 

 of slow-moving (ball or globular) lightning — which are very 

 difficult to correlate with any purely electrical manifestation — it 

 is questionable whether it is possible for the ob'^erver to deter- 

 mine the direction in which the electrical current moves. In 

 ordinary cases the velocity of the electrical discharge is so great, 

 and the duration of the luminous Hash is so brief, that it is im- 

 possible for the unassisted eye to determine the direction of 

 motion. 



According to the experiments of Prof. Rood [American 

 Journal of Scitnce, third serie-, vol. i. p. 15, 1871 ; also idem, 

 vol.v. p. 163, 1873), the duration of lightning-flashe- varies from 

 1/1600 to 1/20 of a second. Even the maximum duration of 1/20 

 of a second is probably too small to be recognised by the unaided 

 human eye. Hence simple observation by means of the eye 

 cannot determine the direction in which the electrical current 

 moves. 



It is nevertheless tme that the eye seems to perceive the direc- 

 tion fof motion of the luminous tract from one point of the 

 cloud-covered sky to another. But this seeming recognition of 

 direction must be an illusion of judgment based upon our inter- 

 pretation of the phenomena presented to the sight. In these 

 cases, our judgment of direction of motion seems to be dependent 

 upon two considerations ; — 



(i) When the flash bifurcates or forlcs, we imagine (probably 

 from the analogy of a ruptured projectile) that the electrical 

 discharge passes in the direction of the diverging branches. 



(2) But the more conmon cause of illusion of judgment in 

 relation to the apparent direction of motion of the electrical 

 discharge arises from the difference of brightness of different 

 portions of the luminous path ; this gives rise to a difterenre of 

 duration of the lingering visual impression on the retina. Thus, 

 in the case of a flash several kilometres in length, one extremity 

 will probably be much nearer to the observer than the other ; 

 and hence the light emanating from one end will traverse a 

 greater thickness of absorbing atmosphere than that emanating 

 from the other end. This would necessarily render one extremity 

 of the luminous path brighter than the other ; and consequently 

 the duration of the impression on the retina would be greater 

 for one end than for the other : hence the flash would seem to reacli 

 the end where the visual effect lingered longer at a later period 

 than the other extremity. In other terms, the light produced in 

 the luminous path is really generated sensibly at the same instant 

 of time along its entire length, and the apparent direction of 

 discharge is an illusion of judgment arising from the varying 

 duration of the visual impression, due to difterences of brightness 

 in different portions of the flash. It is evident that the refine- 

 ments of modern methods of measuring indefinitely small 

 intervals of time might render the actual direction of motion of 

 the electrical discharge appreciable to our senses. 



With regard to the zigzag and irregular branching forms of 

 lightning-flashes, these are the natural results of electrical dis- 

 charges through an interrupted and non-homogeneous medium. 

 The enormous length of some flashes (eight or ten kilometres) 

 indicates that the intervening non homogeneous dielectric acts as 

 an interrupted conductor. In such a medium the path of electrical 

 discharge is along the line of least resistance, which is the line 

 of best induction, which is likewise the line of best conduction. 

 In the atmosphere these lines are irregular and are perpetually 

 shifting, hence the path of discharge may be nearly rectilinear 

 at one time, branching at another time, and even quadrilateral 

 at another time. John Le Conte 



Berkeley, California 



The quotation from M. Him in your issue of January 27 

 (P- 3°3) suggests a few remarks. What may be the greatest 

 length of a flash of lightning ? In the year 1S43 I attempted to 

 answer this question by the following observations. 



My Inarya but had far-projecting eaves supported by rough 

 posts, some black, others white, and thus easy to distinguish. 

 On the first appearance of a storm in a brick-red cloud I took my 

 seat near the tore-hold, leaning my head against the door-post, 

 and holding to my ear a pocket-chronometer. Among several 

 flashes I noticed one nearly horizontal. It travelled northwards, 

 and its thunder followed 54'4 seconds later. The thermometer 

 being then at 19° C, 1 took that degree of heat, from want of 

 better information, as mean heat of the whole trajectory, and got 

 thus 343'7 metres for the velocity of a sound per second. This 

 gave a distance of i8'7 kilometres for the commencement of the 

 flash. It had begun before post A and ende I beyond post D. 

 As they were near me, I took care not to move my head before 

 measuring with a small sextant the horizontal angle between A 

 and D. I found it = 20° 30', and obtained thus 6;6o metres for 

 the length of flash, supposing it horizontal and perpendicular to 

 my line of sight. This result was a minimum, because the 

 angle was evidently too small, and because moreover the flash, not 

 quite horizontal, had travelled obliquely towards me. I drew the 

 latter conclusion also from what appeared to me a fact on this 

 and on other occasions, viz. my ear referred the thunder suc- 

 cessively to different parts of the preceding flash. If an 

 amanuensis had been at hand, I could have dictated to him at 

 what beats of the watch the sound came from the direction of 

 each post. It would then have been easy to get at least a rough 

 estimate of the azimuth in which the flash travelled, and conse- 

 quently its real length. In a similar way I measured on another 

 day a flash more than ten times longer. I have not put down its 

 particulars, because such an enormous result made me fear some 

 mistake in time or angle. On my return to Europe, I mentioned 

 these observations to the late E. Petit, then astronomer at 

 Toulouse. He subsequently informed me that he had measured 

 two flashes of lightning, one 13 and another 17 kilometres long. 

 Should you publish the foregoing note you may induce other 

 observers to follow this line of inquiry with improved appliances. 

 Antoine d'Abbadie 



Abbadia, Hendaye, February 2 



Dr. Modigliani's Exploration of Nias 



You have on two occasions given news of Dr. Elio 

 Modigliani's recent exploration of this remarkable and inter- 

 esting island. I believe it will therefore interest your readers 

 if I endeavour to complete such information. Dr. Modigliani 

 returned to Florence from Nias a short time ago, and at the last 

 meeting of our Anthropological Society gave an able and graphic 

 account of his visit to the island, and especially of his experience 

 of the people ; he illustrated his communication with an exhibi- 

 tion of the rich and very complete ethnological and anthropo- 

 logical collections he has made. 



The natives of Pulo Nias are evidently Malesoid, judging from 

 the numerous interesting photographs taken by Dr. Modigliani, 

 and yet they have peculiarities of their own ; and looking at the 

 fine series of cr;;nia exhibited, one would say that on a Malayan 

 face a Papuan skull had been fastened. Dr. Modigliani found 

 also some resemblance between the Nias people and some of 

 the hill tribes of Southern India. No traces of stone or shell 

 implements are found in use at Nias. The natives get their iron, 

 brass, and gold from traders, principally Chinese, but work the 

 metals themselves with a primitive forge, making axes (hafted 

 in wooden, club-like handles, as those of some African tribes), 

 lance-heads, and swords: the former, usually barbed, recall the 

 Cele! an ones ; the latter are very like \h& parangs of the Bornean 

 Dyaks. Their shields are often heavy and cum irons, coated 

 with buffalo-hide, very Bornean in shape ; they make besides 

 curious iron helmets of a common Asiatic pattern. The swords 

 are sheathed in wood, and have in front a globular wicker or 

 rotang basket, the size of a big orange, which contains curious 

 and very various amulets, with which they never part willingly ; 

 the handle is often carved so as to reprsent a grotesque human 

 face. The Nias people are inveterate head-hunters, and Dr. 

 Modigliani showed one of their ghastly trophies procured whilst 

 he was there, and preserved in spirits. The head is buried, and 

 when the flesh has fallen oft', the skull is hung up under the 

 council-house. Every young fellow to be considered a man 

 must have cut off at least one head— no distinction is made of sex 



