"In the morning ye say, it will be foul weather to-day, for the sky is red and lowering."—Matt. xvi.

528. Why do we have twilight mornings and evenings?

Because the coming and the departing rays of the sun are refracted and reflected by the upper portions of the atmosphere. (See [Fig. 13].)

529. How long before the sun appears above the horizon does the reflection of his light reach us?

The time varies with the refracting and reflecting power of the atmosphere, from twenty minutes to sixty minutes. But the sun's position is usually eighteen degrees below the horizon when twilight begins or ends.

530. Why is the sky blue?

The white light of the sun falls upon the earth without change; it is then reflected back by the earth, and as it passes through the atmosphere portions of it are again returned to us, and this double reflection produces a polarised condition of light which imparts to vision the sensation of a delicate blue. (See [549].)

531. Why do the clouds appear white?

Because they reflect back to us the solar beam unchanged.