An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

38 TTHE E-, ARTH.
persing the solar light, and scattering it in various directions,
no objects would be visible to us out of direct sunshine; every
shadow of a passing cloud would be pitchy darkness; the stars
would be visible all day, and every apartment into which the
sun had not direct admission, would be involved in the obscurity of night. This scattering action of the atmosphere on the
solar light, is greatly increased by the irregularity of temperature caused by the sun, which throws the atmosphere into a
constant state of undulation, and by thus bringing together
masses of air of different temperatures, produces partial reflections and refractions at their common boundaries, by which
means much light is turned aside firom the direct course, and
diverted to the purposes of general illumination.* In the upper regions of the atmosphere, as on the tops of very high
mountains, where the air is too much rarefied to reflect much
light, the sky assumes a black appearance, and stars become
visible in the day-time.
CHAPTER IV.
OF TIME.
99. TIME is -a measured portion of indefinite duration.
Any event may be taken as a measure of time, which divides
a portion of duration into equal parts; as the pulsations of the
wrist, the vibrations of a pendulum, or the passage of sand
from one vessel into another, as in the hour-glass.
100. The great standard of time is the period of the revolution of the earth on its axis, which, by the most exact observations, is found to be always the same. The time of the earth's
revolution on its axis is called a sidereal day, and is determined
by the revolution of a star from the instant it crosses the meridian, until it comes round to the meridian again. This interval being called a -sidereal day, it is divided into 24 sidcereal
hours. Observations taken upon numerous stars, in different
Q Herschel.