142 Meteoric Astronomy. (April, 
particular turn, the means of proving the existence of other 
meteor systems which produce no recurrent showers, or no 
displays so persistently recurrent as to leave us assured of 
their being due to a true meteor system. 
For, if many meteors seen on any night appear to radiate 
from a particular part of the heavens, we can infer very pro- 
bably, though not quite certainly, that those meteors form a 
single group travelling all in the same direction. We can 
infer this, because we know that the observed event would 
certainly happen if the meteors form such a group; but we 
cannot be quite certain that one group only is in question, 
because two meteor groups crossing the earth’s track at the 
same place might have different directions and velocities 
so adjusted that the members of each would seem to 
encounter the moving earth in the same direction. The 
unlikelihood of the coincidence renders the inference that a 
single group is in question so much the more probable. 
Now, if on the same night of other years,—or very nearly 
at the same date,—more meteors are seen to proceed from 
the same radiant point, we conclude that not a group but a 
system is in question, since at the very spot where meteors 
were really travelling in a definite direction in one year, 
other meteors are found following in their track in later 
years. Many millions of miles must separate the first set 
from those seen later; and the inference is, that along the 
whole range of those millions of miles there are meteors 
travelling with equal velocity in one and the same direc- 
tion,—in other words, traversing a definite orbit region. 
Applying this principle, the observers of meteors—Alex- 
ander Herschel, in England ; Professor Newton, in America; 
Secchi, in Italy; Heis, in Germany; Quetelet, in Belgium; 
and Schmidt, in Greece; besides many others—have estab- 
lished the existence of seventy-six distin¢t meteor systems 
having radiant points north of the equator; while 
Neumayer, at Melbourne, and other observers, have noted 
many others having southern radiant points. It may be 
concluded safely that upwards of 150 meteor systems cross 
the earth’s orbital track around the sun. Counting,—as 
resulting probably from the existence of corresponding but 
more important systems,—those falls of fireballs and of 
aérolites which have been observed tu occur on or near cer- 
tain definite days, and taking also into account the pro- 
bability that some meteoric systems have hitherto escaped 
recognition, we may infer that some 200 systems of bodies 
traversing the interplanetary spaces pass close to the 
earth’s orbit. 
