Mat 1. 1894.] 



KNOWLEDGE. 



116 



streams and comets. He showed that the August meteors 

 revolve in an orbit which is, as near as can be determined, 

 identical with the orbit of Comet II. 1862, usually known 

 as Tuttle's comet, and that the November meteors move 

 in an orbit which is practically identical with the orbit of 

 Comet I. 1866, known as Tempel's comet. A few years 

 later it was shown that the Andromid meteors are revolving 

 in a track which is practically identical with the orbit of 

 Biela's comet, and that the April meteors are also 

 associated with a cometary orbit. In view of this 

 evidence, it seems highly probable that the nebulous 

 matter which renders comets visible is evaporated from 

 meteor swarms as they approach the sun, and that the 

 matter thus separated from the meteors is repelled by the 

 sun, never to be regathered by the meteor swarm. It 

 would follow that the meteor swarms which are associated 

 with comets are wasting away and losing a portion of their 

 substance every time that they approach the sun. Possibly 

 some of the particles driven away may fall upon the 

 planets and go to augment their atmospheres, taking the 

 place of carbon, oxygen, and other elements which have 

 been absorbed or added to the solid substance of the planet 

 by the processes of life, oxidation, and other chemical 

 changes continually going on ; and it would seem to 

 follow that comet-producing meteor swarms cannot last 

 for geologic ages, and that those which are now associated 

 with comets must have been introduced into the solar 

 system within a comparatively recent period — a conclusion 

 which is opposed to Mr. Proctor's and Sir Robert Ball's 

 ejection theory, which assumes that comets and meteors 

 had their origin within the solar system, and were ejected 

 from the sun and the larger planets at an early period 

 when these bodies were in a sun-like state. 



The accompanying diagram (page 114) has been copied 

 from a drawing made by Mr. A. G. Sivaslian, of Northfield, 

 Minnesota. It represents Jupiter's family of comets, and 

 was originally published in an interesting article by Prof. 

 W. W. Payne in the October number of his " Popular 

 Astronomy." 



It will be noticed that nearly all the farthest points of 

 these orbits (the aphelia) are on one side of Jupiter's 

 orbit, and that the places where the dotted portions of the 

 orbits join the continuous ones are in nearly every case 

 close to the orbit of Jupiter. The dotted portions of the 

 orbits are below the ecliptic, while the continuous por- 

 tions are to the north, so that one of the nodes of the 

 comet's orbit is in every case near to the orbit of Jupiter. 

 If Jupiter were at that part of his orbit at the time 

 when the comet was passing its node, the two bodies 

 would really be near to each other, and Jupiter by his 

 great mass would have a very marked influence in 

 changing the orbit of the comet. It seems, therefore, 

 probable that all these comets have at some time been 

 considerably perturbed by Jupiter. 



The sun and the whole solar system is moving through 

 space towards a pomt La the constellation Hercules, having a 

 right ascension of about 269°and a north declination of about 

 35°. The bottom of the plate corresponds to right ascension 

 270° ; consequently, a vertical line on the page approximately 

 corresponds to the direction in which the solar system is 

 moving — probably at a rate of some ten miles a second. '■' 

 Jupiter is moving in his orbit with a mean velocity of a 

 httle more than eight miles a second, in a direction con- 

 trary to the hands of a watch when looked at from above 

 the ecliptic, as in the diagram ; consequently, when Jupiter 



* This is the Telocity determined by Prof. Vogel from his observa- 

 tions of the motion of stars in the line of sight. It seems to be the 

 most trustworthy determination of the velocity of the sun's motion in 

 spac* that we at present have. 



is in the part of his orbit which lies on the left hand side 

 of the diagram he is moving very rapidly through space, 

 with a maximum velocity of about eighteen miles a 

 second, while in the part of his orbit which corresponds 

 to the right hand side of the page he is moving in a 

 direction about the sun nearly contrary to the sun's 

 motion in space, and when going slowest he is only 

 moving through space with a velocity of about two miles 

 a second. The distribution of the aphelia of the 

 cometary orbits shows that Jupiter has caught many more 

 comets when moving rapidly through space than when he 

 is moving slowly. But this is only what might be expected 

 if the comets were all originally caught from outside the 

 solar system, and it forms an interesting additional piece 

 of evidence tending to prove that the sun is actually 

 moving in space, and that the comets of Jupiter's family 

 were not originally members of the solar system. 



A similar piece of evidence tends to show that the 

 meteors we encounter were not originally members of the 

 solar system. It has long been known that the earth 

 encounters a larger average number of meteors in the 

 autumn half of the year than in the spring half, from 

 midwinter to midsummer ; that is, when the earth is 

 moving most rapidly through space it encounters fewest 

 meteors. Exactly the contrary of this would be the case 

 if the majority of meteors were sporadic — that is, if they 

 came from outside space. But with meteor streams 

 captured by the larger planets in the way in which Jupiter 

 has annexed his comst family, we should expect to find the 

 earth most involved amongst the perihelia of such elliptic 

 streams on the opposite side of its orbit from that on 

 which the larger planets are most successful in capturing 

 such streams. The observed facts can therefore be best 

 explained by supposing that the majority of meteors are now 

 moving in closed orbits, which are arranged in a manner 

 that shows that meteors as well as comets were originally 

 captured from outside space. 



Eftttrs. 



[The Editor does not hold himself responsible for the opinions or 

 statements of correspondents.] 



MOONLiaHT PHOTOGRAPHS. 

 To the Editor of Knowledge. 



Dear Sib, — It seemed to me that the best way to test 

 your theory as to the markings on Mr. Levingston's 

 photograph, published in Knowledge for December, 1893, 

 would be to repeat the process as an experiment. I 

 therefore recently took the opportunity of a bright moon 

 to expose a plate to a landscape for one hour, and then to 

 point the camera directly to the moon ; the result shows 

 a tracing made by the moon. 



The camera had a focal length of four and a half inches, 

 with an aperture in stop of 0-4 of an inch. 



I forward the plate for you to make use of as you like. 

 Yours truly, 

 Cranleigh, Surrey. J. P. Maclear. 



[Admiral Maclear's photograph shows a band or 

 knotted trace which is very similar in appearance to Mr. 

 Levingston's supposed lightning flash. — A. C. Ranyabd.] 



METEOKS IN THE LUNAR ATMOSPHERE. 

 To the Editor of Knowledge, 



Sir, — In your interesting article on the moon, in the 

 April number of Knowledge, you infer that it is probable 

 the moon has an atmosphere comparable in density with 



