602 



NA TURE 



{Oct. 18, 1888 



NOTES ON METEORITES} 



V. 



'\7^7E shall see next that another line of thought and in- 



. quiry was required to completely establish the cosmical 



hypothesis by giving us data as to the velocities of the meteorites. 



This was that the sporadic meteors, those which made their 

 appearance by chance, so to speak, were always more numerous 

 in the morning than in the evening hours, and further that the 

 numbers seen in the northern hemisphere in one half year was 

 greater than that seen in the other. These facts, although at first 

 theyseemedto connect these phenomena with our terrestrial hours, 

 and therefore were at first considered to militate against the cosmi- 

 cal hypothesis, were subsequently shown, byBompas, A. S. Her- 

 schel, H. A. Newton, and Schiaparelli, to be a distinct proof that 

 the bodies were moving in space with a velocity not incomparable 

 with, but at the same time somewhat greater than, that of the 

 earth itself ; that therefore they were moving with planetary 

 velocities, and therefore were truly members of the solar system. 



The work of M. Coulvier-Gravier 2 was the first to indicate 

 the extreme regularity with which the numbers increased from 

 sunset to sunrise, as will be seen in the accompanying table : — 



Time of 



Observation. 



5 p.m.- 6 p.m. 



6 p.m.- 7 p.m. 



7 p.m.- 8 p.m. 



8 p.m.- 9 p.m. 



9 p.m. -10 p.m. 

 to p.m. -1 1 p.m. 

 II p.m. -1 2 



Number seen 

 per hour. 



7-2 



6-5 

 70 



6-3 

 7-9 



8-o 



9-5 



Time of 

 Observation. 



12 -I a.m. 



1 a.m. -2 a.m. 



2 a.m. -3 a.m. 



3 a.m. -4 a.m. 



4 a.m. -5 a.m. 



5 a.m. -6 a.m. 



6 a.m. -7 a.m. 



Number seen 

 per hour. 



... 107 



... I3-I 



... 16-8 



... 156 



... 13-8 



••• 137 

 ... 13-0 



It was the dependence of these phenomena upon certain ter- 

 restrial hours which made that eminent observer decline to 

 consider their origin as in any way cosmical. 



Mr. Bompas, 3 commenting on the numbers obtained by 

 Coulvier-Gravier, wrote — • 



" The part of the heavens towards which the earth is moving 

 at any time is always six hours from the sun. At 6 a.m. the 

 observer's meridian is in the direction of the earth's motion ; and 

 at 6 p.m. in the opposite. 



" Thus the greatest number of meteors are encountered when 

 the observer's meridian is in the direction of the earth's motion, 

 and the number diminishes from thence to 6 p.m., when he 

 looks the opposite way." 



The accompanying wood-cut will make this clear. The front 

 half of the earth ploughing its way through space is unshaded ; an 

 observer is being carried along the line of the earth's motion at 

 sunrise, the earth is behind him, so to speak, and the point 

 towards which the earth is travelling lies 90° in longitude behind 

 the sun. 



Combining these facts, Bompas explained the results on the prin- 

 ciple that if the meteors be distributed equally in space they 

 would converge to the earth, if at rest, equally on all sides. But 

 if the earth be in motion, and with a velocity one-half the 

 average velocity of the meteors, they would converge to it more 

 on the side towards which it is moving than the other : and in 

 the proportion of nearly two-thirds of the number, would have 

 an apparent motion more or less opposed to that of the earth, 

 and apparently diverging from the point towards which the earth 

 is moving, with a gradual increase in number from 6 p.m. to 

 6 a.m. 



Before we proceed to show the bearing of this matter, a word 

 must be said with regard to the actual conditions under which 

 these bodies reach us from space, and how the fall of these 

 bodies upon the earth and their appearance in the heavens even 

 in the case of no fall have been investigated. 



To approach the proof of the cosmical hypothesis afforded 

 by these observations, we may begin by supposing the earth at 

 rest. If the movements of the cosmical particles are in all direc- 

 tions, they will fall equally on all parts of the earth, and even 

 the earth's rotation will make no difference. .But if we assume 

 the earth's movement in its orbit to be much more rapid than 

 the movements of the meteorites, it is clear that its forward half 

 will receive blows while the hinder half cannot. 



Suppose that all the regions of space swept through by the 

 earth in its orbit round the sun were occupied here and there by 



1 Continued from p. 559. 



2 " Recherches sur les Meteores," p. 219 (Paris, 1859). 



3 Monthly Nances, vol. xvii. p. 148. 



meteorites, also like the earth moving in orbits round the sun, 

 and let us assume for the moment that they are pretty nearly 

 equally distributed and are moving in all directions. 



Under these circumstances the earth in movement in its orbit, 

 at the rate of about 1000 miles a minute, would be sweeping 

 through them all the year round, and we should get the appear- 

 ance of a shooting-star or the fall of a meteorite every day in 

 the year. Careful observations in climates most convenient for 

 these researches, where the sky is freest from cloud and is purest, 

 show, as we have seen, that there is not only no night but no 

 hour without a falling star. We are therefore justified in con 

 sidering that practically the part of the solar system which is 

 swept through by the earth is not a vacuum, not empty space, 

 but space peopled with meteorites here and there. 



If these meteoritic bodies are equally distributed and are going 

 in the same direction as the earth, but moving more quickly, 

 they would follow and catch the earth ; if they were travelling in 

 the same direction as the earth, but more slowly, we should over- 

 take them, and the two sides of the earth separated by a plane at 



Fig. 10. 



right angles to the tangent to the part of the orbit along which it is 

 moving at the time (see Fig. 10) would experience a different con- 

 dition. One side would be bombarded by the greater number of 

 meteorites in the former case, while in the latter the forward 

 half only would be affected. The assumption, however, is that 

 they are travelling in all directions ; hence the numbers which 

 fall on the front hemisphere compared with those that fall on the 

 opposite one — in other words, the numbers seen at sunrise as 

 compared with those seen at sunset — -must depend wholly on the 

 velocity of the earth as compared with the mean velocity of the 

 meteorites. 



The point of space towards which the earth is travelling at 

 any moment, shown in Fig. 10, has been called "the apex of 

 the earth's way " ; the point of space it is leaving the "anti- 

 apex. " 1 



1 These terms were suggested by Prof. Pritchard. In 1866, Schia- 

 parelli suggested point de mire. Quite recently, Prof. Newton, of Yale, 

 has suggested "goal" and "quit." 



