head of the constellation
of the Lion, or "Sickle of Leo," as it is popularly termed, whence
their name--Leonids. It was from a discussion of the observations then
made that the American astronomer, Olmsted, concluded that these meteors
sprang upon us from interplanetary space, and were not, as had been
hitherto thought, born of our atmosphere. Later on, in 1837, Olbers
formulated the theory that the bodies in question travelled around the
sun in an elliptical orbit, and at the same time he established the
periodicity of the maximum shower.

The periodic time of recurrence of this maximum, namely, about
thirty-three years, led to eager expectancy as 1866 drew near. Hopes
were then fulfilled, and another splendid display took place, of which
Sir Robert Ball, who observed it, has given a graphic description in his
_Story of the Heavens_. The display was repeated upon a smaller scale in
the two following years. The Leonids were henceforth deemed to hold an
anomalous position among meteor swarms. According to theory the earth
cut through their orbit at about the same date each year, and so a
certain number were then seen to issue from the radiant. But, in
addition, after intervals of thirty-three years, as has been seen, an
exceptional display always took place; and this state of things was not
limited to one year alone, but was repeated at each meeting for about
three years running. The further assumption was, therefore, made that
the swarm was much denser in one portion of the orbit than
elsewhere,[27] and that this congested part was drawn out to such an
extent that the earth could pass through the crossing place during
several annual meetings, and still find it going by like a long
procession (see Fig. 20, p. 269).

In accordance with this ascertained period of thirty-three years, the
recurrence of the great Leonid shower was timed to take place on the
15th of November 1899. But there was disappointment then, and the
displays which occurred during the few years following were not of much
importance. A good deal of comment was made at the time, and theories
were accordingly put forward to account for the failure of the great
shower. The most probable explanation seems to be, that the attraction
of one of the larger planets--Jupiter perhaps--has diverted the orbit
somewhat from its old position, and the earth does not in consequence
cut through the swarm in the same manner as it used to do.

The other November display