upon
its surface, goes to augment the accumulation of that humble domestic
commodity which men call dust. This continual addition of material
tends, of course, to increase the mass of the earth, though the effect
thus produced will be on an exceedingly small scale.

The total number of meteors moving about in space must be practically
countless. The number which actually dash into the earth's atmosphere
during each year is, indeed, very great. Professor Simon Newcomb, the
well-known American astronomer, has estimated that, of the latter, those
large enough to be seen with the naked eye cannot be in all less than
146,000,000,000 per annum. Ten times more numerous still are thought to
be those insignificant ones which are seen to pass like mere sparks of
light across the field of an observer's telescope.

Until comparatively recent times, perhaps up to about a hundred years
ago, it was thought that meteors were purely terrestrial phenomena which
had their origin in the upper regions of the air. It, however, began to
be noticed that at certain periods of the year these moving objects
appeared to come from definite areas of the sky. Considerations,
therefore, respecting their observed velocities, directions, and
altitudes, gave rise to the theory that they are swarms of small bodies
travelling around the sun in elongated elliptical orbits, all along the
length of which they are scattered, and that the earth, in its annual
revolution, rushing through the midst of such swarms at the same epoch
each year, naturally entangles many of them in its atmospheric net.

The dates at which the earth is expected to pass through the principal
meteor-swarms are now pretty well known. These swarms are distinguished
from one another by the direction of the sky from which the meteors seem
to arrive. Many of the swarms are so wide that the earth takes days, and
even weeks, to pass through them. In some of these swarms, or streams,
as they are also called, the meteors are distributed with fair evenness
along the entire length of their orbits, so that the earth is greeted
with a somewhat similar shower at each yearly encounter. In others, the
chief portions are bunched together, so that, in certain years, the
display is exceptional (see Fig. 20, p. 269). That part of the heavens
from which a shower of meteors is seen to emanate is called the
"radiant," or radiant point, because the foreshortened view we get of
the streaks of light makes it a