1872.] Meteoric Astronomy. 149 
seem insufficient as a basis for such wide generalisations as 
these; but it must be remembered that they are the only 
two cases we have yet been able to deal with satisfactorily. 
It should be remarked, however, that some other comets 
have passed close by the earth’s orbit, and that in several 
cases meteoric radiant points have been recognised which 
correspond fairly with the assumption that those comets are 
followed by meteor trains. However, as nothing has yet 
been proved respecting the relative length of meteoric trains 
following after comets, we cannot expect that the track of 
every comet should to any great distance be thus followed ; 
and where the distance is relatively small, no evidence of 
the kind just referred to would be obtainable even a year or 
two (perhaps) after the comet had crossed the earth’s orbit. 
And here a word or two may be permitted on the question 
of the condition of comets freshly arriving on the scene of 
the solar system. It is assumed sometimes that the train 
of meteors already exists when the comet first comes within 
the solar domain. But, however this may be in other 
cases, there can be no question that in the August and 
November meteors, the train, at least the train we are 
acquainted with, must have been formed after the comet 
had become a member of the solar family. This will be 
evident if we consider how this last-mentioned result can 
alone come about. Take the case of the November meteors, 
the figure of whose orbit shows that the parent comet was 
forced to become a regular attendant of the sun by the dis- 
turbing influence of Uranus. Now, the circumstances 
under which a comet approaching the sun on a parabolic or 
hyperbolic orbit can be thus affected must be regarded as 
exceptional. The planet’s influence must in the first place 
be very energetically exercised ; in other words, the arriving 
comet must pass very close to the planet; for under any 
other circumstances the sun’s influence so enormously 
outvies the planet’s that the figure of the cometic orbit 
would be very little affected. Moreover, the planet’s attrac- 
tion must produce an important balance of retardation. 
The planet will inevitably accelerate the comet up to a 
certain point, and afterwards will retard it; the latter influ- 
ence must greatly exceed the former. ‘To show how greatly 
the comet must be retarded, it is only necessary to mention 
that the actual velocity of the November meteors, when 
they cross the orbit of Uranus, is less than one-third of the 
velocity with which Uranus himself travels; but their 
velocity at the same distance from the sun, when they were 
approaching him from some distant stellar domain, exceeded 
ee 
