449. Why is it supposed that the electrical state of the air and the clouds affects the formation of hail?

Because hail is more common in the summer than at other seasons, and is frequently attended by storms of thunder and lightning.

450. Why do hail-storms most frequently occur by day?

Because the clouds, being charged with vapour to saturation, favour the formation of hail by sudden electrical or atmospheric changes. In the gradual cooling of night, the clouds would expend themselves in rain.

Astonishing facts respecting hail-storms are upon record. In 1719 there fell at Kremo, hailstones weighing six pounds. In 1828 there was a fall of ice at Horsley, in Staffordshire, some of the pieces of which were three inches long, by one inch broad; and other solid pieces were about three inches in circumference. Hail storms are most frequent in June and July, and least frequent in April and October. Hail clouds float much lower in the sky than other clouds; their edges are marked by frequent heavy folds; and their lower edges are streaked with white, the other portions being massive and black. ([Fig. 10].)

CHAPTER XXII.

451. What is light?

Light, according to Newton, is the effect of luminous particles which dart from the surfaces of bodies in all directions. According to this theory, the solar light which we receive would depart from the sun and travel to the earth.

According to Huyghens, light is caused by an infinitely elastic ether, diffused through all space. This ether, existing everywhere, is excited into waves, or vibrations, by the luminous body.