their Appearance, Nature, and Origin. 9 
appeared on the other side of the sun, unlike the sheep, it grew 
a new tail. 
Origin of Comets.—It is probable that comets are bodies floating 
about in interstellar space, and come only accidentally within the 
reach of the solar attraction. 
With regard to the periodic comets, it is supposed that they, 
too, were chance visitors, but that, in their course, they have 
passed near one of the big planets, Saturn or Jupiter, and been 
‘‘captured”’ by them—that is, prevented by the planet’s attraction 
from wandering away again into space. They are thus naturalized 
citizens, not native born members of the solar system. 
Fic. 5.—a, B, Cc, D. VARIOUS PosITIONS OF A COMET NEAR PERIHELION, 
SHOWING DEVELOPMENT AND ORIENTATION OF THE VARIOUS TAILS. 
a. Hydrogen tail. b. Hydrocarbon tail. c. Sodium and Iron tail. 
Fate and End of Comets.—Since comets are continually losing 
part of their substance by means of their tail, it follows that they 
cannot last indefinitely: eventually periodic comets must fritter 
themselves away. 
But it also happens that, even before this occurs, comets break 
up. Thus a comet, named Biela’s comet, was discovered in 1826 
with a period of about 64 years. At its return in 1846 it had 
broken up into two fragments. These two halves were again 
observed at the next return, 1852, since when nothing has been 
seen of Biela’s comet. But on the night of Nov. 27th, 1872, as 
the Earth was passing through the old track of the vanished 
comet, it encountered a brilliant shower of shooting stars. The 
