464 



NA TURE 



[Marc A 19, 1885 



At intervals of six months the earth's motion in space 

 is in exactly opposite directions, and yet these streams of 

 meteors enter the atmosphere from the same apparent 

 radiants. Evidently therefore the meteoric particles, 

 which individually move in parallel flights, are travelling 

 independent of solar attraction and are presented to us 

 under a totally different aspect to the cometary showers 

 the phenomena of which are clearly understood. 



If meteoric streams of great width are encountered by 

 the earth as the result of the sun's proper motion in space 

 then it would appear that to give the phenomena of 

 stationary radiants they must move with enormous veloci- 

 ties. This is not borne out by the observations, for the 

 meteors of these long-enduring streams exhibit appear- 

 ances similar to what is generally observed in the meteors 

 from the cometary showers. The farther the radiant is 

 removed from the earth's apex the slower become the 

 motions of the meteors, they lose the streak-generating 

 capacity, and their colour changes from white to yellow 



or red, indicating a lower degree of incandescence as the 

 result of a less violent friction with the atmosphere. 

 There are exceptions, however, for the meteors from some 

 radiants retain a velocity much greater than that theoreti- 

 cally assigned. 



There is a very pressing need for further observa- 

 tions specially directed to the visible trajectories of shoot- 

 ing stars. The apparent motions of the corpuscles 

 belonging to a stream depend upon several conditions 

 which are very liable to originate discordances. The 

 particles near the radiants move slowly in short courses 

 owing to foreshortening, and when the radiant is near 

 the horizon the nights are longer and more gradual than 

 when it has reached a considerable altitude. The 

 Geminids of December, for instance, appear very slow in 

 the early hours of the evening, but in the morning their 

 swift, diving courses would lead the observer to attribute 

 them to an entirely separate family were it not that the 

 radiant occupies an identical place to that determined 



from the slower meteors recorded several hours before. 

 Some showers also doubtless furnish meteors which 

 become igneous at greater distances than others and 

 more relatively slower than those belonging to streams 

 formed of materials not so readily combustible. More- 

 over the specific gravity of the particles of different 

 systems probably varies to some extent, and their indi- 

 vidual forms may not always coincide, so that the effects 

 of atmospheric resistance must necessarily introduce 

 peculiarities in the observed flights. 



The idea occurred to me that these long-enduring 

 radiants must result from terrestrial meteor streams, i.e. 

 streams revolving around the earth in an excentric orbit 

 with perigee near the outer limits of the atmosphere. If 

 streams of this character existed and were closing in 

 upon the earth we should have the phenomena of station- 

 ary radiants. And the fact of their closing in upon us 

 would be rendered possible on the assumption of a resist- 

 ing medium (similar to that affecting the motion of Encke's 

 comet), or that at each return to perigee the atoms en- 



countered the tenuous outer region of the atmosphere, 



which, though not sufficiently dense to render them 



incandescent, would slightly diminish their velocity and 



thus bring about a contraction of the orbit. But there 



are difficulties to the adoption of such views, one of which 



is that the meteors from such streams would exhibit a 



consistency of velocity whatever the relative position of 



! their radiants with regard to the earth's direction of 



' motion, and this does not accord with the facts of obser- 



! vation. 



The earth's atmosphere probably extends in a barely 

 appreciable degree a much greater distance than ordinary 

 estimates allow. The computed heights of certain meteors 

 deduced from multiple observations, and the phenomena 

 of minute, telescopic shooting stars, which are evidently 

 far exterior to ordinary naked eye meteors, render this 

 highly probable. The former are very numerous, though 

 to what degree is only known to those who have been 

 habitually engaged in sweeping the heavens with a tele- 

 scope of low power and large field. According to my 



