April 4, 1889] 



NATURE 



535 



intention, which, he tells us, is "to place within the reach 

 of all connected with the protection of game a knowledge 

 of the law which it is their duty to administer," and we 

 may hope, also in his words, that it may contribute " to a 

 more effective suppression of a form of lawlessness which 

 leads to so many crimes of a more serious nature." See- 

 ing that it is a second edition, that the author is the head 

 of the constabulary of an important Border county, and 

 that in compiling it he has had the assistance of other 

 chief constables, Mr. Porter is probably right on all points 

 that have been decided. Within his prescribed limits h^ 

 traces his subject, as it seems to us, very well, though 

 tersely enough, beginning with the earliest records of both 

 Kingdoms, and ending with the silly (but well-meant) 

 Sand Grouse Act which closed the labours of the last 

 session of Parliament — an admirable instance of locking 

 the stable-door after the steed was stolen. However, his 

 treatment is so purely from an executive aspect that 

 comments upon the book are hardly suited to these pages. 

 Yet we may remark that the game law question, which, 

 not so many years ago, was a party cry, has through 

 certain modifications of opinion ceased to occupy that 

 position. There are now few reasonable men who do 

 not perceive that if our ever-increasing population is to 

 continue the enjoyment of the delicacies which " game " 

 (using the word in a wide and not a legal sense) affords, 

 some sort of preservation of such " game " is a necessity. 

 The practical extermination, in many large districts, of 

 Lepus timidus since the passing of the Act of 1880(43 

 and 44 V^ict. cap. 47), leading to the almost prohibitive 

 price of hares in our markets, has fully shown this. The 

 way in which the principle of preservation should be 

 applied is, of course, quite another thing. All men 

 nowadays agree in condemning the savagery of the early 

 laws, though many are apt to forget that it was only of 

 a piece with the savagery of other contemporary laws ; but 

 until within the last twenty years few enactments, whether 

 in Great Britain or Ireland, grasped the scientific truth 

 which has been at the bottom of the most recent legisla- 

 tion (the Ground Game Act excepted), and should be at 

 the bottom of all — namely, take care of the parents, and 

 you may leave the offspring to take care of themselves. 



LETTERS TO THE EDITOR. 



[ The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents . Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications. ] 



The Meteoric Theory of Nebulae, &c. 



My last letter in Nature of March 7 (p. 436) substantially con- 

 tended that there must be a certain limit lo the rate of translatory 

 motion possible to meteoric masses, beyond which they would 

 be rapidly resolved into vapour and consequently be unable to 

 maintain the weight of the heaped-up material constituting the 

 nebula (supposed to be formed of meteorites). It appears toler- 

 ably evident that there must be such a limit as to size to which 

 the system can apply, if a period anything like " a few thousand 

 years " mentioned ^ by Prof. G. H. Darwin is to be accorded 

 to this stage of evolution. The doubt was expressed whether 

 the size (or mass) of the original solar nebula was not past that 

 limit, in view of the rate of translatory motion required, viz. 5i 

 kilometres per second in the mean. 



That the theory is in principle true, appears but little open to 

 doubt. For when masses fall in a confused manner to a centre, 

 the kinetic movement is naturally and inevitably produced, and 

 corrected so as to be maintained symmetrically in all directions 

 by the masses themselves. The main question seems to be, 

 What is the limiting mass of the future sun to which such a 



* Abstract of Mr. G. H. Darwin's paper "On the Mechanical Conditions 

 of ft Swarm of Meteorites," appeared in Nature, November 22 and 29, 

 1888 (pp. 8i and 105). The complete paper is in the Philosophical Transactions, 

 vol. cbcxx., 1889. 



system can apply in its completeness, so as to allow anything 

 like permanence enough for the forcible tendency of the energy 

 to equalize itself to extend to a notable radial distance (to settle 

 down into a sort of temporary kinetic equilibrium, that is) ? It 

 is conceivable that in the case of an eventual very large sun, the 

 system of meteorites may be heaped up to a certain height, and 

 then collapse at or near the nucleus, where the density is greatest, 

 and consequently the impacts most numerous. Such a partial 

 collapse would give rise to violent oscillation in the nebula, and 

 perhaps assist the throwing off of rings (query). The fact that 

 meteorites are exceptionally the only celestial masses that we 

 can handle and analyze in our laboratories, gives a basis of 

 certainty to inquiries about them, which lends a special interest 

 to theories as to the part played by them in Nature. 



In my last letter I avoided the use of the term "elasticity," 

 considering purely the physical conditions which constitute its 

 basis. Prof. G. H. Darwin remarks on this point : — " It may, 

 however, I think, be shown that the very greatness of the 

 velocities will impart what virtually amounts to an elasticity of a 

 high order of perfection" (Nature, p. 82). " Ordinary elas- 

 ticity must be nearly inoperative " (p. 107). To produce this 

 exceptional degree of elasticity, the expansive action of the gas 

 generated by the high velocity of impact is relied on, which is 

 regarded to act as " a violent ^ explosive introduced between 

 the two stones." No doubt the volatilized gases due to the col- 

 lision expand and assist rebound ; but in estimating its value we 

 must consider if the chilling of the gas, in doing the work, is at 

 all the equivalent of the heating. If the gas is left in an incan- 

 descent state (as is probable, from spectroscopic evidence also), 

 that represents so much spent work, so much - imperfect elas- 

 ticity. The gas is not confined as in a gun, and so cannot exert 

 its expansive effect to full advantage. 



Meteorites constituting a nebula having a mean velocity of 5^ 

 kilometres per second represent an energy about eighty times 

 that of a cannonade. Taking the velocity of an ordinary pro- 

 jectile at 2000 feet per second, that of the meteorites (5^ kilo- 

 metres) is i8,oDo feet per second, about. Meteorites normally 

 composed of materials imperfectly welded together, cannot stand 

 the same knocking about as steel projectiles. If it were ima- 

 gined that the latter could have a velocity of 18,000 feet per 

 second imparted to them, they would doubtless leave a luminous 

 track in the atmosphere from friction, and the energy of their 

 mutual collision at this speed may be imagined. It must be 

 remembered, however, that the collisions are generally oblique 

 or glancing. 



No doubt in a nebula a metallic rain flying in all directions 

 like the meteorites would accompany the motion. This rain 

 of metal we may suppose under favourable accidents to collide 

 together to form nuclei in various parts of the nebula ; and an 

 incipient nucleus constituting a sort of shelter would collect 

 more rain (or hot metal), and so constitute a new meteorite — 

 much as the occasionally dissociated lumps or molecular clusters 

 of a compound gas reunite in another part of the gas, so that the 

 mean state of aggregation remains the same. 



The mean interval between encounters depends, of course, on 

 the mean size of the meteorites. It appears from the data 

 afforded that thirty days or a month would be the average 

 interval between collisions (at about one-third radius of the sup- 

 posed original solar nebula), for meteorites of mean mass equal 

 to 3I kilogrammes, which would possess a volume of about half 

 a cubic decimetre. 3 So that a nucleus, which we may suppose 

 at rest or only slowly moving— as it is, on the average, struck 

 equally on all sides by the metallic rain — would have, as a 

 mean, a month to collect or grow before the chance of being 

 disturbed by a collision in a nebula under the conditions 

 named. In this way, doubtless, nuclei could be formed, and 

 the meteorites renewed. 

 The question of the applicability of the theory would seem to 



* The entire passage is : " It must necessarily be obscure as to how a 

 small mass of s ilid matter can uke up a very large amount of energy in a 

 small fraction of a second, but spcctrosc ipic evidence seems to show that it 

 does so ; and if s), we have virtually what is a violent explosive introduced 

 between the two stones" (Natuke, November 22, 1888, p. 82). 



' It will be seen afterwards that this radiation must n >t be considered as 

 absolutely lost, but in great part radiated to another region of the nebula of 

 meteorites. 



3 For meteorites of 3^ tonnes (about half a cubic metre in volume), 

 the interval between collisions would be 300 days, nearly a year ; and the 

 mean length of path 130,000,0:0 kilometres, nearly the sim's distance. 

 Multiplying the mass by a thousand (or the diameter by lo) increases the 

 interval between collisions and the length of path ten times. At this rate 

 we should get tj the stellar d.stances and stellar move.nents, regarding the 

 visible universe as a nebula, if a merely speculative reaiark may be allowed . 



