Page:Popular Science Monthly Volume 1.djvu/408

394 is, in the transformation of a cosmical cloud through the influence of the sun, only a question of time; in course of years the matter composing a comet which describes an orbit round the sun must be dispersed over its whole path; if the original orbit be elliptical, an elliptic ring of meteors will gradually be formed from the substance of the comet, of the same size and form as the original orbit.

Schiaparelli has, in fact, discovered so close a resemblance between the path of the August meteors and that of the comet of 1862, No. III., that there cannot be any doubt as to their complete identity. The meteors to which we owe the annual display of falling stars on the 10th of August are not distributed equally along the whole course of their orbit; it is still possible to distinguish the agglomeration of meteoric particles which originally formed the cometary nucleus from the other less dense parts of the comet; thus, in the year 1862 the denser portion of this ring of meteors through which the earth passes annually on the 10th of August, and which causes the display of falling stars, was seen in the form of a comet, with head and tail as the densest parts, approached the sun and earth in the course of that month. Oppolzer, of Vienna, calculated with great accuracy the orbit of this comet, which was visible to the naked eye. Schiaparelli had previously calculated the orbit of the meteoric ring to which the shooting-stars on the 10th of August belong before they are drawn into the earth's atmosphere. The almost perfect identity of the two orbits justifies Schiaparelli in the bold assertion that the comet of 1862, No. III., is no other than the remains of the comet out of which the meteoric ring of the 10th of August has been formed in the course of time. The difference between the comet's nucleus and its tail that has now been formed into a ring, consists in that, while the denser meteoric mass forming the head approaches so near the earth once in every 120 years as to be visible in the reflected light of the sun, the more widely-scattered portion of the tail composing the ring remains invisible, even though the earth passes through it annually on the 10th of August. Only fragments of this ring, composed of dark meteoric particles, become visible as shooting-stars when they penetrate our atmosphere by the attraction of the earth, and ignite by the compression of the air.

A cloud of meteors of such a character can naturally only be observed as a meteor-shower when in the nodes of its orbit—that is to say, in those points where it crosses the earth's orbit—and then only when the earth is also there at the same time, so that the meteors pass through our atmosphere. The nebula coming within the sphere of attraction of our solar system would, at its nearest approach to the sun (perihelion), and in the neighboring portions of its orbit, appear as a comet, and when it grazed the earth's atmosphere would be seen as a shower of meteors.

Calculation shows that this ring of meteors is about 10,948 millions of miles in its greatest diameter. As the meteoric shower of the 10th