June 12, 1890] 



NATURE 



151 



hasty sketch of a few especially important formations that 

 have been closely studied. 



We pass over the oldest deposits, for the interpretation 

 of which but few points of vantage present themselves, 

 and we shall fix our attention on the upper half of the 

 Carboniferous formation, the so-called Coal-measures. It 

 has received this name because in many countries it 

 contains those thick beds of fossil fuel which have become 

 an indispensable factor of modern industries, and without 

 which the actual status of our social and political con- 

 dition could not have been attained. So great is the 

 quantity of the fuel herein stored, that all that is furnished 

 by other geological formations, taken together, falls far 

 short of it. There is much difference of opinion as to 

 the mode in which coal has been formed ; but whatever 

 disagreement there may be in matters of detail, this 

 much is certain, that we have in coal the altered remains 

 of a land-vegetation, which, partly at least, flourished in 

 swamps. Of course the formation is not all or even 

 chiefly coal ; even where it is richest in coal, by far the 

 greater part of the formation consists of shale, sandstone, 

 and conglomerate, and the coal-beds are here and there 

 interstratified, forming but a fraction of the total thickness. 

 We may picture to ourselves the building up of the 

 formation by supposing that a plain or depression was 

 sometimes covered with water, sometimes dried up. 

 When flooded, chiefly with fresh and but rarely with 

 sea-water, beds of shale or sandstone were deposited ; 

 when dry, land or swamp plants sprang up, and their 

 decayed remains furnished the material of coal. Then 

 followed another period of inundation, and thick beds 

 of shale, sandstone, and conglomerate were again de- 

 posited 



The vegetation that in its decay formed our beds of 

 coal was of a peculiar character. As yet there existed 

 no trees (with true foliage) and no flowering plants. A 

 monotonous growth of plants with stiff leaves then 

 clothed our continents. A great part in it was played by 

 Calamites, great plants which no longer exist, and whose 

 nearest relatives are the mares' tails so often met with in 

 marshy ground ; another important type was that of the 

 Lepidodendra, lai'ge trees whose forked stems were covered 

 with leaf scars arranged in a regular geometrical pattern, 

 and the branches of which were clothed with short, 

 stiff, grass-like leaves ; and most important of all, the 

 Stgillarias, the unbranching and twigless stems of which 

 were marked with leaf scars in perpendicular rows and 

 scale-like leaves. Both of these are long since extinct ; 

 and only the insignificant club-mosses of our present 

 flora recall to us the varied gigantic forms of that distant 



age. 



H. F. B. 



( To be continued.) 



LIGHTNING AND THE ELECTRIC SPARK} 



AT a date at least as remote as 600 years b.q. the 

 Greek philosophers were acquainted with a curious 

 little fact to which the modern science of electricity owes 

 its name. They knew that a piece of amber {rjXeKrpov) 

 when rubbed against some suitable substance acquired 

 a temporary attractive power, in virtue of which it became 

 capable of lifting and holding light objects, such as dry 

 leaves or pieces of straw. But another remarkable effect 

 which often attends the friction of amber was for many 

 centuries altogether overlooked. In A D. 1708 it was first 

 noticed by Dr. Wall that a piece of strongly excited amber 

 emitted sparks, which were accompanied by crackling 

 sounds, and these he had the sagacity to compare to 

 thunder and lightning. 



It must be confessed that the recognition of any re- 

 semblance between the microscopic scintillations thus 



' Extracted from a lecture on " Electrical Phenomena in Nature," delivered 

 by Mr. Shelford Bidwell, F.R.S., at the London Institution on February 10, 

 1890. 



NO. 1076, VOL. 42] 



produced and the brilliant lightning flash imposed a some- 

 what severe strain upon even the scientific imagination, 

 and a few years later Stephen Gray, in reference to the 

 same comparison, expressed the hope that " there might 

 be found out a way to collect a greater quantity of the 

 electric fire" than was then possible. His hope was 

 realized by the subsequent improvement of electrical 

 apparatus, and especially by the invention of the Leyden 

 jar ; and the effects obtainable by the means now at our 

 command amply justify the speculations of Wall and 

 Gray. The essential identity of the artificial electric 

 spark with the natural lightning flash was conclusively 

 established by the experiments of Franklin, and in these 

 days it has become a mere common-place, familiar to 

 everyone. 



There are generally said to be two kinds of lightning 

 I flash, which are known as forked lightning and sheet 

 lightning, the former being dangerous and destructive, the 

 latter harmless. To these is sometimes added a third 

 class, called ball lightning. The lightning flash of artists 

 which is familiar to us from innumerable pictures, and of 

 which the venomous-looking zigzag now projected upon 

 \ the screen (copied from an engraving) is a fair example, 

 j has no existence in nature. It is simply an artistic fiction 

 or symbol, like the conventional representation of a 

 galloping horse, which, in the severe language of Mr. 

 Muybridge, resembles nothing to be found in the heavens 

 above or in the earth beneath. The absurdities commonly 

 perpetrated in depicting animals in motion have been 

 fully exposed by Mr. Muybridge with the assistance of 

 photography. So, too, it is photography that has given 

 the cot^p de grdce to the traditional forked lightning. 

 Within the last few years an immense number of photo- 

 graphs of lightning flashes have been made. The 

 Meteorological Society has formed a collection of these, 

 containing about 200 examples, which, by the kindness of 

 Mr. Marriott, the Secretary of the Society, I have had an 

 opportunity of examining carefully. Not a single instance 

 of the artistic lightning flash is to be found among them. 

 The great majority bear a close resemblance to the sparks 

 of our electrical machines : a few are distinguished by 

 peculiarities which, though at first sight a little difficult to 

 account for, can generally be explained and even imitated 

 artificially. 



What may be called a typical lightning flash is a 

 stream of light which follows a sinuous and wavering 

 course, very like that of a river as shown upon a map. 

 [Several photographs of this kind of flash were exhibited by 

 means of the lantern.] The next slide is a photograph of 

 a machine- spark, about 3^ inches in length. The two 

 kinds of discharge are so much alike in their general 

 character that if it were not for the surroundings it would 

 be hard to tell which was the lightning and which the 

 artificial spark. 



The variations upon the normal type of flash, which the 

 Meteorological Society's photographs show, have been 

 classified as ramified or branched lightning, beaded 

 lightning, meandering or knotted lightning, ribbon light- 

 ning, and, lastly, dark lightning. 



Branched lightning is again strikingly suggestive of a 

 river in a map ; not a simple stream, however, but one 

 into which a number of tributaries flow. [Photographs 

 were shown.] Sparks having branches of just the same 

 character are easily produced by a large electrical 

 machine. To obtain theeff'ect well, the negative terminal 

 should be made much larger than the positive, and the 

 two should be separated so far that a spark will only just 

 pass between them. According to Faraday, a ramified, 

 or as he sometimes calls it a "brushy," spark occurs 

 when the whole of the electricity has not been discharged, 

 but only portions of it, more or less, according to circuna- 

 stances. It is a "dilute " spark, generally passing to air 

 or other badly conducting matter (" Exp. Res.," § 1448). 

 When therefore a ramified flash occurs we may reason- 



