Part 13 (1/2)
When diffused matter, precipitated from a rarer medium, is aggregating, there are certain to be here and there produced small flocculi, which, either in consequence of local currents or the conflicting attractions of adjacent ma.s.ses, remain detached; as do, for instance, minute shreds of cloud in a summer sky. In a concentrating nebula these will, in the great majority of cases, eventually coalesce with the larger flocculi near to them. But it is tolerably evident that some of the remotest of these small flocculi, formed at the outermost parts of the nebula, will _not_ coalesce with the larger internal ma.s.ses, but will slowly follow without overtaking them. The relatively greater resistance of the medium necessitates this. As a single feather falling to the ground will be rapidly left behind by a pillow-full of feathers; so, in their progress to the common centre of gravity, will the outermost shreds of vapour be left behind by the great ma.s.ses of vapour internally situated. But we are not dependent merely on reasoning for this belief. Observation shows us that the less concentrated external parts of nebulae, _are_ left behind by the more concentrated, internal parts. Examined through high powers, all nebulae, even when they have a.s.sumed regular forms, are seen to be surrounded by luminous streaks, of which the directions show that they are being drawn into the general ma.s.s. Still higher powers bring into view still smaller, fainter, and more widely-dispersed streaks. And it cannot be doubted that the minute fragments which no telescopic aid makes visible, are yet more numerous and widely dispersed. Thus far, then, inference and observation are at one.
Granting that the great majority of these outlying portions of nebulous matter will be drawn into the central ma.s.s long before it reaches a definite form, the presumption is that some of the very small, far-removed portions will not be so; but that before they arrive near it, the central ma.s.s will have contracted into a comparatively moderate bulk. What now will be the characters of these late-arriving portions?
In the first place, they will have extremely eccentric orbits. Left behind at a time when they were moving towards the centre of gravity in slightly-deflected lines, and therefore having but very small angular velocities, they will approach the central ma.s.s in greatly elongated ellipses; and rus.h.i.+ng round it will go off again into s.p.a.ce. That is, they will behave just as we see comets do; whose orbits are usually so eccentric as to be indistinguishable from parabolas.
In the second place, they will come from all parts of the heavens. Our supposition implies that they were left behind at a time when the nebulous ma.s.s was of irregular shape, and had not acquired a definite rotary motion; and as the separation of them would not be from any one surface of the nebulous ma.s.s more than another, the conclusion must be that they will come to the central body from various directions in s.p.a.ce. This, too, is exactly what happens. Unlike planets, whose orbits approximate to one plane, comets have orbits that show no relation to each other; but cut the plane of the ecliptic at all angles.
In the third place, applying the reasoning already used, these remotest flocculi of nebulous matter will, at the outset, be deflected from their straight courses to the common centre of gravity, not all on one side, but each on such side as its form determines. And being left behind before the rotation of the nebula is set up, they will severally retain their different individual motions. Hence, following the concentrating ma.s.s, they will eventually go round it on all sides; and as often from right to left as from left to right. Here again the inference perfectly corresponds with the facts. While all the planets go round the sun from west to east, comets as often go round the sun from east to west as from west to east. Out of 210 comets known in 1855, 104 are direct, and 106 are retrograde. This equality is what the law of probabilities would indicate.
Then, in the fourth place, the physical const.i.tution of comets completely accords with the hypothesis. The ability of nebulous matter to concentrate into a concrete form, depends on its ma.s.s. To bring its ultimate atoms into that proximity requisite for chemical union--requisite, that is, for the production of denser matter--their repulsion must be overcome. The only force antagonistic to their repulsion, is their mutual gravitation. That their mutual gravitation may generate a pressure and temperature of sufficient intensity, there must be an enormous acc.u.mulation of them; and even then the approximation can slowly go on only as fast as the evolved heat escapes. But where the quant.i.ty of atoms is small, and therefore the force of mutual gravitation small, there will be nothing to coerce the atoms into union. Whence we infer that these detached fragments of nebulous matter will continue in their original state. We find that they do so.
Comets consist of an extremely rare medium, which, as shown by the description already quoted from Sir John Herschel, has characters like those we concluded would belong to partially-condensed nebulous matter.
Yet another very significant fact is seen in the distribution of comets.
Though they come from all parts of the heavens, they by no means come in equal abundance from all parts of the heavens; but are far more numerous about the poles of the ecliptic than about its plane. Speaking generally, comets having orbit-planes that are highly inclined to the ecliptic, are comets having orbits of which the major axes are highly inclined to the ecliptic--comets that come from high lat.i.tudes. This is not a necessary connexion; for the planes of the orbits _might_ be highly inclined to the ecliptic while the major axes were inclined to it very little. But in the absence of any habitually-observed relation of this kind, it may safely be concluded that, _on the average_, highly-inclined cometary orbits are cometary orbits with highly-inclined major axes; and that thus, a predominance of cometary orbits cutting the plane of the ecliptic at great angles, implies a predominance of cometary orbits having major axes that cut the ecliptic at great angles. Now the predominance of highly inclined cometary orbits, may be gathered from the following table, compiled by M.
Arago, to which we have added a column giving the results up to a date two years later.
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Inclinations.
Number of Comets
Number of Comets
Number of Comets
in 1831.
in 1853.
in 1855.
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Deg. to Deg.
From 0 to 10
9
19
19
” 10 ” 20
13
18
19
” 20 ” 30
10
13
14
” 30 ” 40
17
22
22
” 40 ” 50
14
35
36
” 50 ” 60
23
27
29
” 60 ” 70
17
23
25
” 70 ” 80
19
26
27
” 80 ” 90
15
18
19
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Total
137
201
210
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At first sight this table seems not to warrant our statement. a.s.suming the alleged general relation between the inclinations of cometary orbits, and the directions in s.p.a.ce from which the comets come, the table may be thought to show that the frequency of comets increases as we progress from the plane of the ecliptic up to 45, and then decreases up to 90. But this apparent diminution arises from the fact that the successive zones of s.p.a.ce rapidly diminish in their areas on approaching the poles. If we allow for this, we shall find that the excess of comets continues to increase up to the highest angles of inclination. In the table below, which, for convenience, is arranged in inverted order, we have taken as standards of comparison the area of the zone round the pole, and the number of comets it contains; and having ascertained the areas of the other zones, and the numbers of comets they should contain were comets equally distributed, we have shown how great becomes the deficiency in descending from the poles of the ecliptic to its plane.
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Number of
Actual
Relative
Between
Area of
Comets, if
Number of
Deficiency.
Abundance.
Zone.
equally
Comets.
distributed.
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Deg. Deg.
90 and 80
1
19
19
0
11.5
80 ” 70
2.98
56.6