July 23, 1874] 



NATURE 



229 



This tail is always away from the sun. At its origin, 

 near the nucleus, it lies in the prolongation of the ra- 

 dius vector; at a greater distance it is curved backwards, 

 as if it met with some resistance which hindered it from 

 following completely the path of the nucleus. The bent 

 axis of this tail is, however, always situated in the plane 

 of the orbit, and this simple fact accounts for the many 

 varieties of aspect presented to our eyes by these comstary 

 appendages. Comets with straight tails appear such only 

 because the eye of the observer is in the plane of the axis 

 of the tail, i.e. in the plane of the orbit. When the earth, 

 in consequence of its annual motion, is carried from 

 this plane, the curvature of the tail becomes manifest ; it 

 becomes more and more pronounced as the comet, seen 

 at first edgeways, so to speak, shows itself more and 

 more on the Hat, like a scimitar, to the observer. 



When the comet, describing the descending branch 

 of its orbit, reaches perihelion, these phenomena acquire 

 their full development. But when it recedes from the sun, 

 describing the ascending branch of its immense para- 

 bolic trajectory, the tail diminishes, disappears, and gives 

 place to a mere elongation. Soon it again assumes the 

 spherical form; the nucleus, which has gradually lost its 

 brightness, is indicated only by a slight condensation of 

 light at the centre of a globular mass entirely similar to 

 that which was first seen. Finally this rounded nebulosity 

 disappears. 



Upon what scale do these phenomena take place, the 

 immediate cause of which is evidently located in the sun ? 

 What may be tlie dimensions of these nebulosities, of 

 these brilliant nuclei, of these curved tails ? These dimen- 

 sions are assuredly formidable. The comet of 1843 had 

 a tail of 60,000,000 leagues, nearly double our distance 

 from the sun. On the sky that tail was drawn like an 

 immense dash of a brush of 65 degrees of angular ampli- 

 tude. The tail of the famous comet of 181 1 was only 

 40,000,000 leagues : but, on the other hand, the head alone 

 of the comet (250,000 leagues in diameter) was nearly as 

 large as the sun. 



As to Donati's comet, its dimensions were more modest ; 

 its nucleus was 1,000 leagues in diameter, and the head only 

 about 13,000 ; the tail was only about 14,000,000 leagues in 

 length. I had the curiosity to estimate approximately the 

 volume of this small comet, and I found, supposing that the 

 thickness of the tail is equal to its breadth, its volume was a 

 thousand times greater than that of the sun. As in reality 

 the tails are flattened, it will perhaps be necessary to reduce 

 this figure by half. There remains enough to show us 

 that our terrestrial globe, so little beside the sun, is only 

 a point in comparison with these gigantic bodies. 



But, on the other hand, everything proves to us that 

 these bodies contain very little matter in so enormous a 

 volume. A characteristic which is special to them, 

 and which assuredly belongs neither to the planets 

 nor to their satellites, is their almost absolute trans- 

 parency. The stars are seen through the tail of a 

 comet as if the tail did not exist ; they can be seen even 

 through the head, much more dense and more brilliant 

 than the tail. It was for long a question whether the 

 nuclei^, at least, of a com;t would not be opaque and 

 solid like a planet ; but, after examination by the most 

 powerful telescopes, it has always been found to be 

 formed of nebulous layers, more and more dense, always 

 permeable by rays of light. This very simple and alto- 

 gether characteristic fact leads us, by itself, to think that 

 cometary matter must be of extreme rarity, for a mist of 

 some thousands or even of sonte hundreds of metres in 

 thickness suffices to hide the stars, while a thickness of 

 from 10,000 to 1 5,000 leagues of cometary matter scarcely 

 lessens their lustre. Desiring to fix our ideas on this 

 subject, I calculated the mass of Donati's comet, and 

 found that it equalled at least that of a sea of 100 metres 

 in depth, and 16,000 square leagues of superficies. This 

 mass is only a fraction, almost imperceptible, of that of 



the earth. It was almost entirely concentrated in the 

 head of the comet and around in the nucleus ; even sup- 

 posing it uniformly distributed over the whole volume of 

 the tail, there will be found, for the mean density of that 

 appendage, only a value incomparably more feeble than 

 the density of the void approached by our pneumatic 

 machines. But it is not to this rare gaseous residue that 

 we must compire the matter of comets ; it will resemble 



Fig. 4- 



rather those impalpable grains of dust which dance in the 

 air, and which are disclosed to us by the smallest ray of 

 solar light penetrating a darkened chamber. • 



Although comets show us matter rarefied to such an 

 extent that a celebrated physicist, M. Babinet, could with 

 considerable justness call them "visible nothings" {riens 

 visibks), do not, however, imagine that their contact 

 with our earth would be without inconvenience. If 

 the nucleus of our comet had directly encountered 

 the earth, with its mass of 25,600 millions of millions of 

 kilogrammes, and its relative speed of seventeen leagues 

 per second (seven for the earth and ten in an opposite 

 direction, for this retrograde comet), the actual energy of 

 the shock would be enormous ; I calculated that its trans- 

 formation into heat would immediately generate fifty-one 

 million calories per square metre of the hemisphere which 

 sustained the shock. It would be enough to shatter, dissolve, 



and volatiliac a ptrt of the solid crust of our globe. No 

 living being could survive such a catastrophe. Happily 

 the probability of such an encounter is excessively small ; 

 and, indeed, the most remote geological ages do not bear 

 any traces of such an adventure. We cannot, however, 

 forget that meteors and shooting stirs, perhaps even the 

 aerolites which bombard us so regularly every year and 

 every day of the year, have probably the same origin as 

 comets, and result from a miss of analogous materials 

 which are decomposed in penetrating our solar world. 



{To be continued.) 



