86 



• KNOWLEDGE • 



[Dec. 2, 1881. 



original rock, it will, if sulijecUcl to the same influences, 

 yield the snmc products wlicn docomposfd ; lying exposed, 

 lis it does, to tlio rain and air, the slow chaiif^es al>ove 

 described take place, and we finally li:ivc a mixture of sand, 

 clay, potash, lime, Ac To aid this slow action, which at 

 the present day is not yet complete, the fanner plou-^hs his 

 tield, which, hrinying up fresh soil from helow, exposes it to 

 the air and rain. We have before staU'd that the fertility 

 of a soil is dependent upon the minerals of the soil and the 

 extent of their decomposition. If, therefore, no aid is given to 

 the solvent action of rain by ploughing, no great amount of 

 fertility can be expected. The variety of felspar, whether it 

 be a soda or a lime felspar, is of great impoi-tiinc^e — the second 

 kind, when decomposed, producing tolerably fertile land, 

 the former not so. When mica is present in large quanti- 

 ties, extreme infertility is the result, and for reasons before 

 stated. It would occupy too much space to allude to all 

 the various circumstiinces productive of the various classes 

 of soils, and we therefore jiroceod to account for the pro- 

 duction of the humus, or vegetable matter, which gene- 

 rally gives the dark colour, for a soil destitute of humus 

 would be of varying .shades of yellow and red. It has 

 been proved by experiment that plants can grow 

 without humus, or vegetable mould, and we need 

 only refer to the growth of lichens on stones, ifci . , 

 as an example ; therefore, the first crop which grew 

 on the new soil, whate\er that crop or its origin 

 might be, would flourish to a cei-tain extent, and, 

 dying, leave its remains behind ; these would rendi r 

 the growth of the next generation of plants more 

 vigorous ; these, in their turn dying, the supply of 

 humus would gradually increase. It might l>e sup- 

 posed that the formation of humus would in time 

 become excessive, l>ut, as a rule, this is not so, 

 because humus gradually decomposes, and in do 

 composing supplies carbonic acid to the soil, so 

 aiding in the decomposition of the minerals present. 

 In all these changes we see the marvellous useful 

 nessof that gas which, under certain circumstances, 

 is so fatal to man ; but here we find it as a most 

 useful servant, for not only does it supply air-food 

 to the plant, being absorbed by the leaves, but, 

 entering into the earth with the rain, it there pre- 

 pares that food which the plant must have, and 

 which it absorbs by its roots. 



COMETS AND COMETS' TAILS. 



By the Editor. 



BEFORE we proceed to consider the theory by which 

 alone, so far as can be judged at present, the ])henoniena 

 of comets' tails can be explained, it may be well that we 

 should consider the evidence derived from other comets 

 than those hitherto considered. 



In the first place we would direct special attention to 

 the comet of 181 1. In this comet, as may be seen from its 

 j)icturc in Fig. 1, the various parts of the comet and its 

 tiiil could be distinguished by the naked eye. There was 

 the condensed part, called the itttcli'i'.g, which in this case 

 was apparently gloVmlar in form ; the nebulous envelope 

 which .surrounds the nucleus, the so-called romfi : the bright 

 side parts of the tail where it seems to be swe)it away from 

 the coma, leaving a comparatively dark region behind the 

 head, and the tail, widening and growing fainter with 

 distmce from the head. No one, we think, who considers 

 this picture will for a moment iniiigine that the comet is a 

 mere lens, and its tail merely the track of light condensed 



by this lens along the region behind the head. Here, 



again, the hollow structure of the tail seems indicated by 

 th(- bright tracks on either side, though, as we shall en- 

 deavour to .show lat<-r, the exceedingly well-defined nature 

 of the dark track Ix-liind the nucleus in many comets seems 

 to force uj)on us a difi'erent int<:rpretation of this singular 

 and characteristic feature. 



In some respects the comet of 1811 tells us more of 

 cometic poFsibilitie."*, so to speak, than any other comet 

 that has ever yet been observed. Discovered on March 26, 

 1811, this comet remained visible for a longer time than 

 any yet seen, viz., for 16 months, 22 days. It had a tail 

 120 millions of miles in length, and 15 millions of miles in 

 diameter at the widest part The diameter of the nucleus 

 was about 127,000 mOes, that of the envelope round the 

 head about 643,000 miles. But what was so remarkable 

 about this comet was, that it obtained this remarkable 

 development without approaching the sun, as other comets 

 have done. The usual rule with comets is that the nearer 

 they approach to the sun, the more their heads and tails 

 are developed. But the least distance of the comet of 

 IHll from the ruti was little less than 100 millions of 



Fig. 1. Comet of 1^11 



miles. Again, although it made so remarkable an appear- 

 ance, as seen from the earth, the distance of that comet 

 from us was at no time less than 110 millions of miles. 

 Its true magnitude, therefore, as Professor Kirkwood well 

 remarks, "has probably not been sui-passed bj- that of any 

 other comet which has j-et b(>en observed.'' If its path had 

 carried it nearer to the sun, its appearance would probably 

 have been terrible in the extreme. If we consider the 

 enormous volume occupied by this comet and its tail, its 



Million cubic miles of hen>l, 

 Ten billion leagues of tail, 



we shall see that the phenomena we have to interpret 

 ought not to escape us in \irtue of minuteness of scale, 



Next consider the great comet of 1861. This comet 

 was discovered on May 1.3, by Mr. John Tebbutt, jun., of 

 New South Wales, and first accurately observed at the 

 Sydney Observatory, on May 26. It passed northwards 

 from the southern skies, and first became visible in Europe 

 in the last week of June, 1861. The first recorded obser- 



