54 



NA TURE 



[May 17, 1900 



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 Naturb. 

 No notice is taken of anonymous communications. '\ 



Percussion Caps for Shooting in Schools. 

 The extraordinary explosive power of fulminate of mercury 

 is known to all chemists, but it is not generally known that the 

 explosion of a percussion cap on a gun will cause a current of 

 air sufficient to extinguish a candle at a distance of ten or fifteen 

 feet. The distance, of course, varies with the length and bore of 

 the gun, and with the nature and the size of the candle. The 

 gun must be pointed at the lower part of the wick, and in order 

 to blow out the candle the aim at this distance requires to be 

 nearly as accurate as would be required to make a centre with a 

 rifle at a hundred yards. In a speech to the Primrose League 

 on May 9, Lord Salisbury mentioned the expediency of every 

 man having the chance to learn to handle a rifle within reach of 

 his own cottage. By beginning with percussion caps children 

 might be taught to handle a gun at such an early age, that, in 

 case of invasion of this country, boys of fourteen might be able 

 to act as soldiers, as they are said to be doing amongst the Boers 

 at the present time. The objections to training children to 

 handle a rifle are, first of all, the danger of the child shoot- 

 ing either itself or some one else ; and secondly, the expense. 

 But the inclination of children to play soldiers might readily be 

 utilised by teaching them to handle first of all a toy gun, and then 

 to practice shooting at a candle with caps. For those who 

 shoot best with caps, the practice with a saloon rifle might be 

 held out as a reward. One single-barrelled old muzzle-loading 

 gun would suffice for many children, and as 240 caps cost a 

 shilling, the expense of providing a gun and material for practice 

 would be very small. Lauder Brunton. 



Escape of Gases from Planetary Atnnospheres. 



In Nature of March 29 (p. 515), Dr. Stoney, in referring to 

 a paper by the writer in the January number of the Astro- 

 physical Journal, raises the question as to the correctness of 

 the use of Maxwell's distribution of velocities in computing the 

 escape of gases from the earth's atmosphere. He maintains 

 that this distribution does not hold at its attenuated limits. In my 

 paper I have not taken conditions which may exist there, but 

 boundary conditions, which are much more favourable to the 

 escape of the molecules of a gas, and certainly compatible with 

 the kinetic theory, if we are to accept such a theory at all. 



Of the four conditions discussed in my paper, I will only refer 

 to the third, the data for which are based on direct observation, 

 namely, - 66° C. at a height of 20 kilometres (the mean of several 

 ascensions really giving - 65° C. to - 70° C. for a height of only 

 16 kilometres). The pressure is calculated from the usual ex- 

 ponential formula, which agrees closely with observations to 

 this height. At these temperatures and pressures there can be 

 no question as to the validity of the kinetic theory. 



Let us assume now that the atmosphere abruptly terminates 

 at this height, and at this temperature the loss would certainly 

 be greater (in fact, very much greater) than under the actual 

 conditions, where the temperature and pressure are much lower. 

 It should also be noticed that in my tables I have assumed the 

 atmosphere to be entirely made up of one gas — for example, 

 helium or hydrogen. Even then only 2673 x io""2*c.c. of helium 

 would escape in 10' years. Hence the assumption that helium 

 is now escaping from our atmosphere is without foundation. In 

 the case of a hydrogen atmosphere only 054 c.c. will escape in 

 one year. If the total amount of air in the atmosphere be taken 

 approximately at 10''^"' c.c, and if the actual density and tem- 

 perature at the outer limits of the atmosphere be also considered, 

 It will be evident how baseless the supposition is that either 

 helium or hydrogen is escaping. It should be further noted that 

 Maxwell's distribution of velocities from zero to infinity is the 

 only one giving a sufficient velocity for any escape at all, Clausius' 

 theory not being adequate. 



It was the assumption that helium is escaping from the at- 

 mosphere — since it had not been detected — that first led me to 

 verify it on the kinetic theory of Maxwell. The discovery, by 

 Ramsay, of helium as a constituent of our atmosphere only tends 

 to confirm the results of my calculations of the impossibility 

 of its escape. S. R. Cook. 



Physical Laboratory, University of Nebraska, April 26. 



NO. 1594, \'OL. 62] 



Racket Feathers. 



Your able reviewer of Meyer and Wiglesworth's " Birds of 

 Celebes " (Nature, April 26), criticising the arguments used to 

 account for the formation of the racket tail feathers of the parrot, 

 Prioniliiriis (as an inherited effect of mechanical attrition on 

 objects against which the tail is liable to be brushed — boughs, 

 walls of nesting-hole, &c.), asks the pertinent question, why so 

 few exposed feathers, such " as the external rectrices and remiges 

 of all birds, and specially the lengthened feathers of wedge- 

 shaped tails (Dicrurus) are neither bare nor racket-shaped nor 

 incipiently so." The insignificant length of the outer rectrices 

 of Dicrurus perhaps safeguards them ; when these feathers are 

 longer, as in the closely-allied Bhringa and Dis<;emurus, they 

 are racket-shaped. As to the remiges and rectrices of birds 

 generally, one feather overlies and to a great extent protects the 

 next ; but still, the outer webs are always very much narrowed 

 in the outermost and most exposed feathers, less narrowed in the 

 next, and so on till in the middle of the wing and tail (where 

 they are well protected on both sides) they are not narrowed at 

 all. But, while normal wing and tail feathers are exposed to 

 attrition on one web only, long feathers standing well out fron:> 

 the rest are liable to have the web frayed on both sides of the 

 shaft as far as they project beyond the other feathers, and to 

 some extent where they rest upon the other feathers through 

 friction against the latter. It is assumed that at some period 

 earlier in the history of the race these elongated feathers were 

 of the usual simple shape, but they are now known to issue from 

 the follicles displaying peculiarities which are often much the 

 same as those obtained by scraping an ordinary feather with a 

 knife — namely, if the shaft is stiff afid not very long, a small 

 terminal spatule is formed (as in Prioniturus, Parotia) ; if the 

 shaft is long and weak, a large spatule (as in Tanysiptera, 

 Loddigesia). A difficulty, perhaps, to the acceptance of the 

 theory is its apparent consequence — that epidermal (in a sense, 

 dead) structures, like feathers, possess the power of transmitting 

 mutilations to posterity. For my own part, I think that the 

 modification of shape of the feathers is communicated to the 

 sensitive tissues (much in the same way as the shape of a stick 

 placed in the hand of a blind man is comprehended by him after 

 touching other things with it), and that a corresponding physio- 

 logical adjustment is made and gradually inherited. The result 

 is probably not an exact recapitulation of the mutilation, but it 

 sometimes appears to be very nearly so. 



L. W. WiGLESWORTH. 



Castlethorpe, Stony Stratford, April 30. 



Mr. WiGLESWORTH in the above note hardly does more than 

 recapitulate the (?) arguments advanced in the "Birds of 

 Celebes." He does not offer any explanation of the crucial 

 difficulties indicated in the review ; why "mechanical attrition 

 on objects," or by the wind, is effective only in so few cases 

 throughout the class Aves when so many species are subject to 

 the necessary conditions ; why, for instance, the species of Palae- 

 amis (belonging to the same sub-family as Prioniturus), or those 

 of the genus Irissor, do not conform to the "law" ; and why 

 one sex of a species may have "sabre wings," or spatulate orna- 

 ments in various situations, and the other sex not. 



The question may also be asked ap7-opos of Mr. Wigleworth's 

 statement above, why in Paradisea rubra the long and weak- 

 shafted tail feathers have the sjnall spatule (which eventually 

 vanishes) instead of a large one, if the knife-scraping Analogy 

 hold.e good ? 



The reasons for the exceptions to the author's rule is what 

 chiefly demands an explanation, in the opinion of 



The Reviewer. 



THE APPROACHING TOTAL ECLIPSE OF 

 THE SUN. 



THE approaching total solar eclipse, on the 28th of 

 the present month, promises to contribute some 

 valuable additions to our scientific knowledge of the 

 centre of our system, inasmuch that the track of the moon's 

 shadow on the earth's surface passes, to an unusual ex- 

 tent, through regions which are easily accessible. Enter- 

 ing the North American continent near New Orleans, 

 in Louisiana, the central line of eclip«se traverses the 

 States of Mississippi, Alaban)a, Georgia and Carolina^ 



