Page:Popular Science Monthly Volume 87.djvu/226

222 say that since the Earth actually passes through at least five prominent meteor swarms, there ought to be thousands of invisible swarms within our solar system which we do not pass through. Newton's investigations led him to the conclusion that about 90 per cent. of the meteors which have encountered the Earth and have been observed with sufficient accuracy to let us determine their orbits are moving around the Sun in eccentric orbits of short periods, like those of the short-period comets, and in the west-to-east direction.

The certainty of rapid disintegration of the periodic comets—extremely rapid in comparison with astronomical time-intervals—is all but equivalent to saying that the periodic comets have been recently captured by our planets; for the periodic comets which we are still observing could not have been following their present orbits during many centuries, except at the price of disintegration to the point of total disappearance.

The zodiacal light is a closely related subject. The phenomenon is due to the presence of countless small particles of solid matter varying perhaps from dust particles up to bodies perhaps many cubic inches or even larger in volume, which scatter the sunlight falling upon them. The volume of space occupied by this finely divided material is very great. It extends north from the Sun to a distance of the order of 100,000,000 kilometers, and there is no reason to doubt an equal southern extension, for observations made in the west after sunset and in the east before sunrise indicate that the structure is symmetrical with respect to the Sun. Its extent in the principal plane of the solar system in all directions from the Sun is even greater. In such clear skies as exist on the tops of mountains the zodiacal light can be seen to stretch entirely across the sky as a faint band following the ecliptic; and this is proof abundant that the materials which scatter the light extend beyond the Earth's orbit.

Inasmuch as we do not distinguish the individual particles which make up the zodiacal light materials, we can not now say whether they are revolving around the Sun from west to east, but we can not doubt the fact that they are revolving around the Sun and that the orbits of a large proportion of the particles are necessarily in planes highly inclined to the general plane of the solar system. Seeliger is of the opinion that this material supplies the attracting mass which disturbs the motion of Mercury, and to a lesser degree the motions of Venus, Earth and Mars. If this be true the total mass of the particles must approximate to that of the planet Mercury.

Where this material came from, whether it is a remnant of the original material which formed the inner planets and the Sun, or whether it has come in from the outer confines of the Sun's sphere of