108

PHYSICAL PROPERTIES OF LIGHT.

SECT. II.

PHYSICAL PROPERTIES

OP

LIGHT.

l. VELOCITY OF LIGHT.

The velocity of light is much greater than that of sound, forthe flash of a gun fired at a considerable distance, is seen beforethe report is heard.

The clap of thunder is not heard till some time after thelightning has been seen, except the phenomena takes place at nogreat distance.

The motion of light is so very rapid that in all common ex-periments it appears to be instantaneous. Its velocity cannot bemeasured between the tops of the most distant mountains thatare visible from each other. Astronomers have neverthelessfound that it may be measured when it moves between some ofthe heavenly bodies. For by the eclipses of Jupiter's satellites,it appears that it takes S minutes and 13 seconds in passingacross the semidiameter of the earth’s orbit. From these ac-curate investigations they have deduced that light passes with avelocity of about 167,000 geographical miles, in one second, orthat it passes from the sun to the earth in little more than eight;minutes.