400 



NATURE 



\Feb. 2 1, 1889 



NOTES ON METEORITES.^ 



VIII. 



'T'HERE can be little doubt that it is to the varying conditions 

 ■*■ produced by the outflows in both directions along the 

 radius vector, to which reference was made in the last article, 

 that the various appearances put on by the axis of comets' 

 tails are due. Thus, in Coggia's comet, to take an instance, the 

 perihelion passage of which took place on August 27, on June 10 

 the axis was brighter than the rest of the tail, but by July 10 the 

 bright axis was replaced by one of marvellous blackness, which 

 was one of the features of the comet at that time, and this dark 

 axis expanded as perihelion was approached. 



The tail is always curved, but if the earth lie in the plane of 

 the orbit the curvature cannot be seen. 





Fig. 27.— Great comet of 1861, seen on June 30, when the earth was 

 plane of the orbit. 



Fig. 28. — Same comet seen on June is. 



The accompanying woodcuts will explain how the solar 

 repulsion produces this curvature, and how the curvature will 

 depend upon the velocity due to repulsion. 



Fig. 29. — Slight repulsion ; great curvature. 



Fig. 29, which I owe to M, Faye,^ represents the successive 

 positions of a series of molecules emitted by the nucleus of 

 a cornet so as to constitute the axis of the tail. A density is 

 imagined such that the repulsive force exactly counterbalances 

 the solar attraction : thus their motion, solely due to the 

 tangential velocity of the comet, takes place in a straight line. 



' Continued from p. 236. 



^ " Forms of Comets," Nature, vol. x. p. 268. 



To again simplify matters, this rate is supposed constant, as if 

 the orbit were a circle. 



On the first day, the comet being at c\ a molecule rn>^ is 

 detached and- subsequently follows the line ;«^ ni'- m^. On 

 the second day, a molecule ;«2, likewise leaves the nucleus at 

 c^, and subsequently describes the tangent mP' m"- wl Similarly, 

 on the third day, for a molecule m^ ; and so on. If we join by 

 a continuous line the series of positions occupied at the same 

 time, the fifth day, by all these molecules, m^, m*, m^, ni^, tn\, 

 we shall have the curvilinear axis of the tail ; this will be in 

 this particular case, the involute of a circle. This construction 

 accounts for the three laws which have been ascertained as the 

 result of observation: (i) the tail, at its origin, is sensibly 

 opposed to the sun, s ; (2) the tail is curved backwards on its 

 path ; (3) the axis of the tail is a plane curve situated in the 

 plane of the orbit. 



If the density of these molecules were still smaller, the repul- 

 sive force would prevail over the solar attraction, and the 

 molecules would describe no longer straight lines, but sections 

 of an hyperbola whose convexity would be turned towards their 

 common focus, s (see Fig. 30). 



Fig. 30.— Here the velocity is greater and the tail is straigliter. 



The series of points w^, iti^, m^, ;«■*, emitted at c', c^ c^, c* 

 by the comet, gives a curve like the former one, but with a 

 curvature much less pronounced and nearer to the radius vector. 



Now the single tail we have been considering will depend 

 upon the repulsive action upon molecules of similar density, 

 and that very small. 



But suppose there are in consequence of collisions among the 

 members of the swarm, several gases given off which can retain 

 their gaseous form, and suppose they are of different densities. 

 Then it is evident that a winnowing process will be set up, and 

 that the molecules of smallest density will be repelled with the 

 highest velocity ; and given these varying densities, we must get 

 more tails than one — one, in fact, for each representative density. 



M. Bredichin, of the Moscow Observatory, has in fact shown 

 that there are three distinct types of tails. In the first class, 

 the tails are long and straight, and the repellent energy of 

 the sun upon the small particles is about twelve times as great 

 as the energy of his gravitational attraction. The particles 

 therefore leave the nucleus with a high velocity, generally about 

 fourteen or fifteen thousand feet per second. The greater this 

 velocity in relation to the rate of travel of the comet, the 

 straighter of course will be the tail, because the particles forming 

 It do not lag behind. In the second type, the energies of the 



