The zodiacal light is probably identical with the meteor called _trabes_ by _Pliny_ and _Seneca_. It was noticed in the latter part of the sixteenth century by Tycho Brahé, who "considered it to be an abnormal spring-evening twilight." It was described by Descartes about the year 1630, and again by Childrey in 1661. The first accurate description of the phenomenon was given, however, by Cassini. This astronomer supposed the appearance to be produced by the blended light of an innumerable multitude of extremely small planetary bodies revolving in a ring about the sun. The appearance of the phenomenon as seen in this country is represented in Fig. 2. [Illustration: Fig. 2.] For general readers it may not be improper to premise the following explanations: Meteors are of two kinds, _cosmical_ and _terrestrial_: the former traverse the interplanetary spaces; the latter originate in the earth's atmosphere. _Bolides_ is a general name for meteoric fire-balls of greater magnitude than shooting-stars. The _period_ of a planet, comet, or meteor is the time which it occupies in completing one orbital revolution. The motion of a heavenly body is said to be _direct_ when it is from west to east; and _retrograde_ when it is from east to west. _Encke's Hypothesis of a Resisting Medium._--The time occupied by Encke's comet in completing its revolution about the sun is becoming less and less at each successive return. Professor Encke explains this fact by supposing the interplanetary spaces to be filled with an extremely rare fluid, the resistance of which to the cometary motion produces the observed contraction of the orbit. METEORIC ASTRONOMY. CHAPTER I. SHOOTING-STARS. I. The Meteors of November 12th-14th.