Snow is found to be of greater importance to man than is generally supposed. But, although in this country we are enabled to recognise the hand of Providence in the gift, there are latitudes wherein the blessing thus conferred is more deeply felt. In such countries as Canada, Sweden, and Russia, the falling of snow is looked for with glad anticipations, quite equalling those which herald the "harvest-home" of England, or the "vintage" of France. No sooner is the ground covered with snow, than cranky old vehicles that had been jolting over rough roads, and sticking fast in deep ruts of mud, are wheeled aside, and swift sledges take their place. Towns distant from each other find an easy mode of communication; the markets are enlivened, and trade thrives. Snow supplies a kind of railroad, covering the entire face of the country, and sledges glide over it, almost with the speed of the locomotive. 447. _What is sleet?_ _Sleet_ is snow which, in falling, has met with a _warmer current of air_ than that in which it congealed. It therefore partially melts and forms a kind of _wet snow_. 448. _What is hail?_ _Hail_ is also the _frozen moisture of the clouds_. It is probably formed by _rain drops_ in their descent to the earth, meeting with an _exceedingly cold current of air_ by which they become _suddenly frozen into hard masses_. It is also supposed that the _electrical_ state of the air and of the clouds influences the formation of _hail_. [Verse: "If the clouds be full of rain, they shall empty themselves upon the earth."--ECCLES. XI.] 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?_