156 



♦ KNOWLEDGE ♦ 



[Aug. 21, 1885, 



stage — a period dui-iiig -nliicli no additional nutriment 

 is taken — is elaborated the gorgeous hue Tvliiclx glorifies 

 its adult form. 



Longicoms are frequently very variable in size, as is 

 usually the case with wood-boring insects. Imprisoned 

 as they are in a burrow which, as larvae, they never 

 leave, they have very little power of selection of food, 

 and are, therefore, entirely dependent upon the supply 

 into the midst of which their excavating labours carry 

 them, and according to the quality of this will be the 

 vigour, or otherwise, of their constitution, and the stature 

 to which they will attain. 



(To he continued.) 



THE GEEAT EED SPOT OX JUPITER. 



Br Richard A. Pkocioh. 



{Continued fro) 



). 106.) 



THE spot was manifestly a surface feature, in this 

 sense that the layer of clouds in which the spot 

 appeared as a sort of opening was part of the visible 

 surface of the planet. This was shown by the circum- 

 stance that as the spot drew near to and passed the edge 

 of the planet its outline remained distinctly visible. Had 

 the spot been due to some formation lying below the 

 surface clouds of the planet, the spot could not have 

 been thus traced up to the planet's edge. Of course this 

 need not prevent us from recognising the true cause of 

 the spot as existing far below the STirface-level ; but 

 manifestly the cloud layer was laid open at its outer sur- 

 face. Now this being so, it is clear that were the forces 

 which formed the spot all at work at that same surface 

 level, and all acting from or towards a centre, we should 

 expect them all to act with about the same degree of 

 force, and the spot to have therefore a circular shape, 

 unless we can recognise some likelihood that in different 

 latitudes on Jupiter different conditions would exist, or 

 in other words unless we can recognise the existence of 

 zones on the planet akin in some sense to the trade and 

 counter-trade wind zones on the earth. 



But although the most characteristic feature of Jupiter 

 is the existence, almost always (if not always), of parallel 

 bands or zones of clouds, diverse in their light-reflecting 

 qualities, these zones have no permanent position like th( 

 trade zones on the earth. They vary almost capriciously 

 in position. Sometimes there are but four or five ' 

 them, at others there are ten or twelve cr even mo 

 We cannot recognise any permanent difference, then, in 

 the condition of the various latitudes on Jupiter, to ac- 

 count for the oval figure maintained so long (six years at 

 least) by the great spot. 



Yet we may still, or rather we must obviously, asso- 

 ciate the lengthening of the great opening in a direction 

 parallel to the cloud zones, with the forces to which the 

 existence of those clouds — as such — is due. Now it has 

 always seemed to me that as the trade wind theory, once 

 complacently advanced to explain the parallel belts of 

 Jupiter and Saturn, most manifestly fails, we are driven 

 to another interpretation of the cloud belts which is 

 very significant in regard to Jupiter's condition. The 

 trade winds and counter trade winds, and the zones 

 named after them, owe their existence to the difference 

 between the rotation velocities in tropical regions which 

 lie farther from the earth's axis and in temperate and 

 arctic regions which lie nearer to that axis. The cloud- 



belts of Jupii tr iind Saturn must also be due to differences 

 of rot;ui.:;:il \ . 1- riiy.— not however between places in 

 different l;ii iiii'l. > . -u tliuse planets, but between regions 

 at ditVd-. i;t I 1. \,iti-ii> in the cloud envelopes of Jupiter 



and S,',:'. W , f. n-l tr, :,linit, seeing that 



the li^:- : ; ! I ■ ' . ,y of accounting 



for tin ' ~, , :, - ._. us, that there 



must 1< :■. :■','■ ' HI,-, nf :i,-rM 111 ;iii,l desccut, in the 

 clouil-Ii i i > . : ;. - Mirrounding the giant planets. 

 Mattir. n U, as columns of ascending vapours, 



or nii-- It' enormous heights from Jovian. 



volctiiii"-, [ I -;;,_ :,- >uch matter does from regions near 

 the centre, where the motion of rotation is slower, to- 

 regions higher up, where the motion of rotation is more 

 rapid, lag behind and cause a trailing of cloud forms 

 towards the west. On the contrary, matter descending, 

 as torrents of falling rain, or matter falling back after 

 ejection, would rusli forwards and cause the cloud forms 

 to be extended towards the east. Granted a stiflicient 

 range in height, whether in ascent or descent, and the 

 parallelism thus arising would be as marked as we 

 actually find it in the cloud-belts of the giant planets. 



But here certain questions arise which we must dispose 

 of before we consider in this light the lengthening of 

 the great spot. Can we imagine that the cloud-laden 

 envelopes surrounding the giant planets have the 

 enormous depth which this explanation would assign to 

 them ? The depth essential for this interpretation must 

 bear a measurable propoition to the diameter of the 

 planet. Less than at least a thousand miles (only a. 

 fortieth of the planet's diameter) would certainly net- 

 suffice ; for obviously the rotational velocities at the top 

 and bottom of a cloud region one thousand miles high 

 on Jupiter would not differ by more than about one^ 

 fortieth, or about 2i per cent., whereas the sharp paral- 

 lelism of the belts "indicates quite a considerable diffe- 

 rence of velocities. Taking, however, even a depth of 

 one thottsand miles for an atmosphere which at its 

 highest part bears clouds such as exist in ottr very 

 highest cloud-bearing atmospheric strata, say ten miles 

 above the sea-level, we find a very remarkable state of 

 things at a depth of even but a hundred miles below the 

 visible cloud surface of Jupiter, unless we sujjpose the 

 laws connecting density and pressure to be very different 

 on Jupiter from the laws recognised here, — a supposition 

 which must not uimecessarily be introduced. For our 

 air ten miles above the sea-level has a density equal to 

 about one-eighth the density of the air we breathe, the 

 density doubling for each 3^ miles (or thereabouts) of 

 descent. Taking this density as that existing at the 

 outskirts of the visible cloud-envelope of Jupiter, we find 

 that with his known (and well-measured) gnivitating 

 energy, the density would double for each mile and a 

 half of descent. But say that it doubles only once for 

 two miles of descent. Then four miles below the visible 

 surface of the planet the atmospheric density would 

 be already half (instead of one eighth) of our air's at 

 the sea-level ; six miles below it would equal the 

 density of our air ; eight miles below it would be double ; 

 and only ten miles below the visible surface of Jupiter 

 the density of his aii- would be four times the density of 

 the air we breathe. After that, in the next ninety 

 miles of descent, taking tis only one htindred miles 

 from the surface, there would be forty-five doublings of 

 pressure and density, making the density millions of 

 millions of times greater than that rf air, thousands of 

 millions of times y-reater than that of water, and htmdreds 

 of millions of times greater than the density of any 

 terrestrial element. This of course is altogether prepos- 



