Page:Popular Science Monthly Volume 1.djvu/349

Rh in 1860 so astonished the Scientific world, afforded decided indications of its presence.

Dr. Weiss, of Vienna, regards comets as the original bodies by whose disintegration meteor-streams are gradually formed. In this respect his views differ somewhat from those of Schiaparelli. "Cosmical clouds," he remarks, "undoubtedly appear in the universe, but only of such density that in most cases they possess sufficient coherence to withstand the destructive operation of the sun's attraction, not only up to the boundaries of our solar system, but even within it. Such cosmical clouds will always appear to us as comets when they pass near enough to the earth to become visible. Approaching the sun, the comet undergoes great physical changes, which finally affect the stability of its structure: it can no longer hold together: parts of it take independent orbits around the sun, having great resemblance to the orbit of the parent comet. With periodical comets, this process is repeated at each successive approach to the sun. Gradually the products of disintegration are distributed along the comet's orbit, and if the earth's orbit cuts this, the phenomenon of shooting-stars is produced."

The characteristics of the different meteor-streams afford interesting indications in regard to their relative age, the composition and magnitude of their corpuscles, etc. etc. Thus, if we compare the streams of August 10th and November 14th, we shall find that the latter probably entered our system at a comparatively recent epoch. We have seen that at each return to perihelion the meteoric cluster is extended over a greater arc of its orbit. Now, Tuttle's comet and the August meteors undoubtedly constituted a single cluster previous to their entering the solar domain. It is evident, however, from the annual return of the shower during the last 90 years, that the ring is at present nearly if not quite continuous. That the meteoric mass had completed many revolutions before the ninth century of our era is manifest from the frequent showers observed between the years 811 and 841. At the same time, the long interval of 83 years between the last observed display in the ninth century, and the first in the tenth, indicates the existence of a wide chasm in the ring no more than 1,000 years since.

The fact that the meteors of the November stream are diffused around only a small portion of their path, seems to justify the conclusion that the transformation of their orbit occurred at a date comparatively recent. Leverrier has calculated that the meteoric cloud passed very near Uranus about a. d. 126. He regards it, therefore, as highly probable that this was its first approach to the centre of our system.