kind, appearing upon a
photograph taken by the astronomer Witt, at Berlin, in 1898, which first
informed the world of the existence of Eros.

[Illustration: PLATE XIII. MINOR PLANET TRAILS

Two trails of minor planets (asteroids) imprinted _at the same time_
upon one photographic plate. In the white streak on the left-hand side
of the picture we witness the _discovery_ of a new minor planet. The
streak on the right was made by a body already known--the minor planet
"Fiducia." This photograph was taken by Dr. Max Wolf, at Heidelberg, on
the 4th of November, 1901, with the aid of a 16-inch telescope. The time
of exposure was two hours.

(Page 227)]

It has been calculated that the total mass of the asteroids must be
much less than one-quarter that of the earth. They circulate as a rule
within a space of some 30,000,000 miles in breadth, lying about midway
between the paths of Mars and Jupiter. Two or three, however, of the
most recently discovered of these small bodies have been found to pass
quite close to Jupiter. The orbits of the asteroids are by no means in
the one plane, that of Pallas being the most inclined to the plane of
the earth's orbit. It is actually three times as much inclined as that
of Eros.

Two notable theories have been put forward to account for the origin of
the asteroids. The first is that of the celebrated German astronomer,
Olbers, who was the discoverer of Pallas and Vesta. He suggested that
they were the fragments of an exploded planet. This theory was for a
time generally accepted, but has now been abandoned in consequence of
certain definite objections. The most important of these objections is
that, in accordance with the theory of gravitation, the orbits of such
fragments would all have to pass through the place where the explosion
originally occurred. But the wide area over which the asteroids are
spread points rather against the notion that they all set out originally
from one particular spot. Another objection is that it does not appear
possible that, within a planet already formed, forces could originate
sufficiently powerful to tear the body asunder.

The second theory is that for some reason a planet here failed in the
making. Possibly the powerful gravitational action of the huge body of
Jupiter hard by, disturbed this region so much that the matter
distributed through it was never able to collect itself into a single
mass.

[18] Sir William Herschel was the first to note