KNOWLEDGE & SCIENTIFIC NEWS. 



[January, 1907. 



times l)y iiii- ciiirinj^ violent tliiiiKU-rslornis. One, tlipii, 

 sees the lif^htnint; travel si"\('ral tiriu's I):n-k\\ aids aiul 

 lorwaids; the oseiilalions must, in consetiiieiice, he 

 somewhat slow, otherwise tiie eye could not cleteet them 

 in(li\ idiially. l'rohahl\- sueh disrharpes aceount for an 

 observation made hv Oove (i'oiji;. Ann. 35, pp. .'?7<)-3So, 



Photo ly Mr, J. 1\ St-lvman, 

 Flir. 6.— An eartli-ta-cloud discharge. Berkhamsted, July, 11:04. 



''^.kS). from which he contluded that such Hashes were 

 intermittent discharges. ' ' 



The above extract shows that Prof. Kayser not only 

 disag-rees with Prof. 'I'ait's subjecti\e origin of the 

 observation of the direction of flashes mentioned in a 

 previous paragraph, but appears quite convinced tiiat 

 oscillati.ry discharges take place. It may be remarked, 

 however, that Dove gave no indication of the direction 

 of travel of the discharges, but simply termed them 

 "intermittent," .so that the flash he observed might 

 have been compo.sed of either all cloud-to-earth or all 

 earth-to-cloud flashes. 



So far as I am aware, the oscillatory nature of a 

 lightning flash has still to be proved. 



1 his question of oscillation is, however, of great im- 

 portance, especially when lightning conductors are in 

 consideration. Thu.s we read (" Modern Lig-htning 

 Conductors," Killing-worth Hedges, 1905, p. 11) that 

 '■ although it has not been conclusively proved that 

 lig-hlning is oscillatory, it is easier to explain its action 

 on this supposition, and the reason whv ordinary con- 

 ductors are so frequently liable to lateral discharge, is 

 that they are constructed on the incorrect a.ssumption 

 that lig-htning obeys the same laws as a constant electric 

 current." 



Again Mr. Alexander G. McAdie, in " Lightning and 

 the Electricity of the Air" (Bulletin No. 26, U.S. 

 Weather Bureau No. 197, p. 30, 1899), wrote: — 



" In the past four years we have learned, through 

 I lie work of Hertz and others, that when an electric 

 current flows steadily in one direction in a cylindrical 

 wire its intensity is the same in all parts of the wire; 

 but if the current be of an oscillatory character, i.e., a 

 <-urrcnt which rapidly reverses its direction, the condi- 

 tion no longer holds, and if the alternations are very 

 rapid the interior of the w^ire may be almost free from 

 current. If lightning, then, be a discharge of an 

 oscillatory nature, it may happen that the current down 

 the hghtnmg rod would be only sitn deep." 



These two extracts indicate' very forcibly that this 

 question of oscillation of the di.scharge is well worth 

 mquiring into, 



It is, however, not proposed here to consider electri- 

 cal oscillations which have such small periods of about 

 the order of a millionth or so of a second of time, but 

 simply those whose p<ri()ds are about one hundredth of 

 a second. 



In the first place it is important to draw attention to 

 the fact that we Icnow that what sometimes appears to 

 the eye as a single flash is in many cases a scries of 

 separate discharges. This fact has been clearly shown 

 to be the case by photographing such flashes with a 

 moving camera. 



Now, what we wish to know is, are such discharges 

 oscillatory; that is, if the components Ix' photographed, 

 arc the flashes cloud-to earth and eartli to-cloucl dis- 

 charges alternately? 



Here, again, I think the same method of investiga- 

 tion, namely, the moving camera, may help us, and we 

 may be enabled to detect whether this alternate reversal 

 of the direction of the current does or does not occur. 



It has been indicated above that the ramifications of 

 a lightning flash branch off the main stream in the 

 direction of travel of the flash. If, therefore, flashes 

 oscillate, as stated above, then should not the separate 

 images of the ramifications from each individual main 

 flash (photographed on a moving plate) he directed 

 alternately downwards and upwards ? 



Photo In, Hen Wall,!-. 



Fig. 7.— A multiple lightninfr flash, showing the ramifications of the 

 separate discharges, all directed towards the eartn. Ham- 

 burg, May 30, 1902. 



Now what do photographs taken in this manner 

 inform us? In every single case, and a whole host of 

 " multiple " flash photographs has been examined, 

 the ramifications are all turned towards the earth; that 

 is, all the individual flashes were cloud-to-earth dis- 

 charges. 



