Aug It si 12, 1 8 So] 



NATURE 



341 



miles per second, takes to pass over a few feet, is required 

 to prove to us that lightning is not absolutely instan- 

 taneous. Wheatstone has shown that it certainly lasts 

 less than a millionth part of a second. Take this, along 

 with Swan's datum, which I have just given you, and you 

 see that the apparent brightness of the landscape, as lit 

 up by a lightning flash, is Lss than one hundred thousandth 

 part of what it would be were the lightning permanent. 

 We have thus rough materials for instituting a comparison 

 between the intrinsic brightness of lightning and of the 

 sun. 



Transient in the extreme as the phenomenon is, we 

 can still, in virtue of the duration of visual impressions, 

 form a tolerably accurate conception of the form of a 

 flash ; and in recent times instantaneous processes of 

 photography have given us permanent records of it. 

 These, when compared with photographic records of ordi- 

 nary electric sparks, bear out to the full the convictions at 

 once forced by appearances on the old electricians, that 

 a flash of lightning is merely a very large electric spark. 

 The peculiar zig-zag form, sometimes apparently almost 

 doubling back on itself, the occasional bifurcations, and 

 various other phenomena of a lightning flash, are all 

 shown by the powerful sparks from an electric machine. 

 [These sparks were exhibited directly ; and then photo- 

 graphs of some of them were exhibited.] 



The spectroscope has recently given us still more con- 

 vincing evidence of their identity, if any such should be 

 wanted. 



The bifurcations of a flash can puzzle no one who is 

 experimentally acquainted with electricity, but the zig-zag 

 form is not quite so easily explained. It is certainly 

 destroyed, in the case of short sparks, by heating the air. 

 [Photographs of sparks in hot and in cold air were 



exhibited. One of each kind is shown in the woodcut. 

 The smoother is that which passed through the hot air. 

 The other passed through the cold air nearer the camera, 

 and is therefore not quite in focus.] 



Now heating in a tube or flame not only gets rid of 

 motes and other combustible materials but it also re- 

 moves all traces of electrification from air. It is pos- 

 sible, then, that [the zig-zag form of a lightning flash 

 may, in certain cases at least, be due to local elec- 

 trification, which would have the same sort of effect 

 as heat in rarefying the air and making it a better 

 conductor. 



A remark is made very commonly in thunderstorms 

 which, if correct, is obviously inconsistent with what I 

 have said as to the extremely short duration of a flash. 

 The eye could not possibly follow movements of such 

 extraordinary rapidity. Hence it is clear that when 

 people say the)- saw a flash go upwards to the clouds 

 from the ground, or downwards from the clouds to the 

 ground, they must be mistaken. The origin of the mis- 

 take seems to be a subjective one, viz., that the central 

 parts of the retina are more sensitive, by practice, than 

 the rest, and therefore that the portion of the flash which 

 is seen directly affects the brain sooner than the rest. 

 Hence a spectator looking towards either end of a flash 

 very naturally fancies that end to be its starting-point. 



(To be coniiniied.) 



OBSERVATIONS ON ARCTIC FOSSIL FLORAS 



WITH REGARD TO TEMPERATURE 

 'T'HE first feelings of surprise caused by the discovery 

 ■'■ of remains of warmer-temperate, sub-tropical, and 

 even tropical plants within the Arctic circle, of, geo- 

 logically speaking, comparatively recent age, have now- 

 died away, and we no longer find that their presence 

 there forms so favoured a theme for speculation. The 

 time appears to have arrived when we may critically 

 examine the botanical evidence upon which estimates of 

 the past degree of warmth enjoyed by the Arctic regions 

 have to be formed. The method open to us is very 

 simple : we have, it seems, only to first set aside deter- 

 minations that are clearly little more than guesses ; then 

 ascertain the minimum mean temperature required by the 

 remaining groups of plants to flourish at the present day ; 

 and the sum of these temperatures should furnish reliable 

 results for each period. 



I am not yet able myself to carry this inquiry beyond 

 the ferns and conifers, but the determinations of these 

 are probably so very much more accurate than those of 

 the higher orders of plants as to comprise most of the safer 

 data, and they are sufficiently numerous for the purpose. 



My present remarks are limited to the Komeschichten. 

 a horizon supposed in the " Flora Fossilis Arctica," to 

 represent in Greenland the Urgonian or Neocomian of 

 Central Europe. In this Komeschichten two genera of 

 ferns occur which deserve especial consideration, for Prof. 

 Heer makes use of their presence to infer that at that 

 period the Arctic regions were favoured with a sub-tropical 

 or even tropical climate. These genera are Gleichenia 

 and Ohandra. The correctness of the determination of 

 the suppo'-ed Arctic Oleandra is doubtful, and it is best 

 for the present to place them among the guesses. The 

 very sparse indications of sori are not satisfactory, and 

 there are no less than twelve widely-distinct genera pos- 

 sessing species with approximately the same venation. 

 Oleandra is a small genus with but six species, almost 

 confined to the tropics, but two of them grow in Northern 

 India at altitudes of 6,000 and 7,000 feet. 



It is quite otherwise with the remains of Gleichenia, for 

 these preserve every characteristic of that genus. But 

 while it is perfectly obvious that these are really fragments 

 of Gleichenias, neither the number of species into which 

 Prof. Heer has divided them, nor the inferences as to 

 climate which he draws from them, can be admitted. He 

 has quite unnecessarily, it seems to me, separated the 

 fragments from the Komeschichten into fourteen species, 

 and to these has added two from the Ataneschichten. The 

 prevailing species, G. Zippei, if considered to represent 

 the type m its average size, might be made to embrace 

 eicfht or ten of them without even then approaching the 

 limits of variation seen in the corresponding existing 

 species. G. Giesekiana receives the rather larger pinnse 

 and G. gracilis the smaller, and many others seetn sepa- 

 rated on trifling or fancied peculiarities, as G. acjitipennis, 

 which is merely a small, indistinct fragment, with a few 

 rounded depressions, conjectured to mark sori, but which, 

 from their position on the mid-rib, could not w^ell be such. 

 Gleichenia is a particularly variable fern. Berkeley men- 

 tions (Introd. to "Crypt. Bot.," p. 516, Fig. \\o,b) that 

 he had seen at Kew the minute pinnules of one of them 

 expanded to three times its normal length, and the 

 margins unfolded by exposure to a warm damp atmo- 

 spheie. In two full-grown specimens of G. dichotoma 

 from Khasia, in the Kew Herbarium, the longest pinnules 

 respectively are one and nine centimetres in length. The 

 Arctic species are, however, closely represented by G. 

 glauca{G. longissima, \-loo\i., " Syn. Filicum"), and in 

 this species the pinnules in different plants vary, from a 

 single locality, between 25 and 2 mm. in length. In 

 making species out of fragments of fossil plants the greater 

 or less liability of the living forms to vary should, it seems 

 to me, be kept in mind, and for general convenience the 



