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.