222 THE POPULAR SCIENCE MONTHLY 



say that since the Earth actually passes through at least five promi- 

 nent meteor swarms, 3 there ought to be thousands of invisible swarms 

 within our solar system which we do not pass through. Newton's in- 

 vestigations 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 — ex- 

 tremely 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 



The zodiacal light is a closely related subject. The phenomenon 

 is clue to the presence of countless small particles of solid matter vary- 

 ing 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 south- 

 ern extension, for observations made in the west after sunset and in the 

 east before sunrise indicate that the structure is symmetrical with re- 

 spect 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 be- 

 yond 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 



s Numerous minor streams are reported by meteor observers. 



