1872.] Meteoric Astronomy. 155 
let the following reasoning be carefully noted :—The 
members of a meteor system in travelling towards their 
perihelion open out their ranks, as it were, because travelling 
there with continually increasing speed, so that a given 
time difference between the place of two meteors corresponds 
to a continually increasing distance. We overrate this 
opening out in taking it as proportional to the square of the 
distance from the sun. Now the volumes of spherical 
spaces around the sun vary as the cube of the distance 
within which they are enclosed. Hence, ceteris paribus, the 
richness of meteoric distribution around the sun being pro- 
number . . 
——-, Must vary inversely as the distance. 
volume 
Thus, a set or group of meteors, which at a distance of go 
millions of miles (about equal to the earth’s), would spread 
with a certain degree of richness, would at a distance of 10 
millions of miles be spread nine times more richly. Now, 
the above table shows that at a mean distance of Io mil- 
lions of miles (taking this as corresponding to the limits 
o and 20 millions,—an assumption very unfavourable to the 
argument) we have a density of perihelion distribution 
represented by 8°65 as against a density of only 0°34 at 
a distance of go millions of miles. Thus the density of 
perihelion distribution is about 25} times greater at the 
former distance; and the actual mean meteoric density 
is about (g x 254, or) 230 times greater. Further, the illu- 
mination of these meteoric bodies at the lesser distance is 
81 times greater, since this illumination varies as the square 
of the distance. Hence, under equally favourable conditions, 
the total quantity of light reflected from meteors within a 
given considerable space, at a distance of to millions of 
miles, exceeds that reflected from a set within an equal 
space at the earth’s distance, in the proportion of 230 x 8i 
to I, or upwards of 2000 times. Nearer to the sun than 
this still enormous distance the quantity of reflected light 
must be vastly greater; and if any meteors become incan- 
descent owing to the great heat to which they are exposed, 
the total amount of light from these sun-surrounding regions 
must be yet further increased. 
It should be noticed that the only assumption which has 
been made in the above reasoning is so far in accordance 
with the evidence actually obtained that any other assump- 
tion would have a considerable weight of probability against 
it. For if Schiaparelli’s discovery has any cosmical import- 
ance at all,—and every one admits that it has,—it implies 
that all comets are followed by meteoric trains. 
portional to 
