402 



NATURE 



\Feb. 2 1, I : 



intense light from time to time arise in those portions of the tail 

 that were furthest from the nucleus. Sometimes instantaneous, 

 and appearing upon a small extension of the extremity of the 

 tail, which then became more visible, the fugitive gleams 

 reminded me of the pulsations of the aurora borealis. At other 

 times they were less fleeting, and their propagation in rapid suc- 

 cession could be followed for some seconds in the direction of 

 the nucleus near the extremity of the tail. These appearances 

 then resembled the progressive undulations of the aurora 

 borealis, but even in this case they were only visible in the last 

 third of the length of the tail. The gleams in question were 

 similar to those that I remember to have seen in the tail of the 

 great comet of 1843, and which were observed by very many 

 astronomers.'" 



The American observers of Donati's comet in 1868 described 

 a number of brighter bands "like auroral streamers" crossing 

 the tail and diverging from a point between the nucleus and 

 the sun.^ 



This point is one well worthy of subsequent inquiry. I have 

 brought together evidence to show that in the aurora one of the 

 chief factors in the production of the spectrum is meteoric dust. 



If this be conceded, we have meteoric dust in all probability 

 very slowly falling through our atmosphere at a height at which 

 its pressure is very low, the luminosity both of the dust and the 

 atmosphere being produced by electricity. Whether the elec- 

 tricity is produced by the movement is a matter on which at 

 present we are quite ignorant, but if it be eventually shown that 

 all aurorse follow well-recognized star-showers a certain amount 

 of plausibility will be accorded to the notion. 



However this maybe, we must in the case of the aurora regard 

 the permanent gases in the air as a constant, and the dust as the 

 variable. 



But if we wish to assimilate these displays with comets' tails, 

 we must in the latter case consider meteorites in space as the 

 constant, and the permanent gases repelled from the comet as 

 the variable. 



Prof. Tait, assuming that the head of a comet is a swarm of 

 meteorites or stones, varying in size from a marble to boulders 

 20 or 30 feet in diameter, has shown that all the various 

 cometary phenomena may be explained. His researches have 

 not yet been printed ut extcnso, but the following general state- 

 ment gives a summary of the results of his calculations which 

 appeared in Good Words some time ago. 



Firstly, with regard to the masses of the comets. The total 

 mass of a comet cannot be very great, for, as we have seen, no 

 measurable disturbance of planetary orbits has been known to 

 be produced, and this small mass is just as likely to be due to 

 scattered solid masses as to one continuous gaseous mass, and 

 indeed we know that this is so. In the case of comets of but small 

 masses, the component meteorites would be small and far apart. 

 Then with regard to the transparency of the comet, it is cal- 

 culated that a meteorite 25 feet in diameter at a distance of half 

 a million miles from us could not totally eclipse a star of the 

 same size as our sun, even if it were at such a distance as to be 

 barely visible to the naked eye. Again, if some of the meteorites 

 were large enough to eclipse the stars behind the comet, the 

 eclipse would be of very brief duration, and we should see the 

 star as if nothing had happened. In order for the comet to re- 

 duce the light of a star seen through it by one-tenth, it would re- 

 quire to be 300 miles thick, supposing the stones to be i inch 

 cube and 20 feet apart. 



While the swarm which builds up the comet is coursing round 

 the sun as a whole, the individual members will themselves 

 gravitate towards each other ; and if we suppose the whole mass 

 to be i/iooo that of the earth, and the meteorites to be uniformly 

 distributed in a sphere 20,000 miles in diameter, those coming 

 from the outside to the centreof the group would have a velocity 

 of about 500 feet per second. The stones colliding will generate 

 heat, and some gas will be evolved ; some members of the mass 

 will be quickened, while other constituents of the mass will be 

 retarded in their motion, and in this way we have a probably suffi- 

 cient explanation of the various forms which the telescope has 

 revealed to us. And then finally Prof. Tait goes on to show that 

 the result of these collisions would be such a smashing up of the 

 constituents of the swarm that much finely-attenuated material 

 would be left behind, sufficient to reflect sunlight, and to give 

 rise to the phenomena of the tail. 



Webb, p. 197. 



If in the imaginary swarm the mass of each stone be 100 

 pounds, and its velocity, due to attraction, be 500 feet per second, 

 the heat resulting from the impact of two of them would be 

 quite sufficient to volatilize a portion, and to make the outsides of 

 the stones white-hot. Stones of this weight would be about 10 

 inches cube, and in the swarm considered there would therefore 

 be about 136,000,000,000,000,000,000 of them. At the rate of 

 one collision per second, there being about 31,436,000 seconds 

 in a year, there would be a possibility of one collision per second 

 for 2, 150,000,000,000 years. There would therefore be material 

 for such collisions for a period of over two million years even at 

 the extravagant rate of one million per second, and on the 

 assumption that no stone comes into collision with another 

 more than once. 



The whole mass being i/iooo that of the earth, and the space 

 occupied being 250 times that occupied by the earth, the stones 

 in question being 10 inches cube will only occupy about 1/8000 

 of the space through which they are distributed ; the average 

 distance apart would be about 17 feet. The swarm would reflect 

 about half as much sunlight as a slab of the same material in the 

 same place, but it would probably be too opaque to transmit 

 starlight. By making the stones larger, and thus increasing the 

 distances between them, the luminosity would be retained, while 

 at the same time the swarm would be sufficiently transparent. 

 It thus seems to suit the hypothesis better if we regard the 

 separate stones to be greater than 10 inches cube. 



J. Norman Lockyer. 



( To be continued. ) 



THE FORCES OF ELECTRIC OSCILLATIONS 

 TREATED ACCORDING TO MAXWELLS 

 THEOR Y. BY DR. H. HER TZ} 



I. 



Note by the Translator. 

 ""PHE early part of the following paper is no doubt familiar io 

 ^ the more important persons in this country, and therefore 

 need perhaps hardly have been translated. Nevertheless, as 

 these experiments of Hertz form a sort of apotheosis of Max- 

 well's theory, it is natural to reproduce this portion as well as 

 the rest ; and inasmuch as Hertz did not at first express his dis- 

 coveries in Maxwellian language, it is interesting to see how he 

 regards the matter since his conversion, and how he now presents 

 his ideas to foreigners. 



I have translated the paper because it seems to me a remark- 

 able example of clear theoretic insight in conjunction with great 

 experimental skill, because it is pleasantly written, and because 

 it deals in a powerful manner with a profoundly interesting 

 subject. 



1 can hardly hope to have escaped errors in translating, but 

 the original paper in Wiedemann' s Annalen for January of this 

 year is very accessible. Oliver J. Lodge. 



The residts of the experiments on quick electric oscillation 

 which I have carried out appear to me to lend to Maxwell's 

 theory of electrodynamics an ascendancy over all others. _ At 

 first I interpreted these experiments in terms of older notions, 

 seeking to explain the phenomena in part by means of the co- 

 operation of electrostatic and electro-magnetic forces. To Max- 

 well's theory in its pure development such a distinction is foreign. 

 I wish, therefore, now to show that the phenomena can also be 

 explained in terms of Maxwell's theory without any such dis- 

 tinction. If this attempt succeeds, questions about special 

 propagation of electrostatic force, being meaningless in Maxwell s 

 theory, are at once settled. And besides this special aim, a closer 

 insight into the play of forces concerned in rectilinear oscillations 

 is not without interest. 



The Formula. 



In what follows we have only to concern ourselves with 

 forces in free ether. Let X, Y, Z, be the components of electric 

 force acting on the points x, y, z ; let L, M, N be the corresponding 

 components of magnetic force ; let t be the time, and let A stand 

 for x/(mK). Then, according to Maxwell, the time-rate of 

 change of the forces is dependent on their distribution in space 

 in the following way : — 



^ Translated and communicated by Dr. Oliver Lodge. 



