Page:Popular Science Monthly Volume 37.djvu/556

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selves to one stone as a nucleus, and then the mass so formed has received external coatings of ice. The compound structure of such stones becomes manifested when the mass gradually thaws. In some cases these stones are coated with crystals of ice in six-sided prisms and pyramids, as perfectly formed as the specimens of quartz or calc-spar crystals which are to be seen in mineral collections. It is very hard to believe that such beautifully formed crystals as these can be the product of any instantaneous process of formation.

It is these heavy blocks of ice which do the greatest amount of damage, as naturally a lump, weighing even an ounce, is a formidable missile when it falls from a height of even a thousand feet. When these falls are about to take place, observers have reported that a peculiar rattling sound is heard in the atmosphere, evidently from collisions between these stones striking one another in their fall. A very careful observer, who was overtaken by one of these falls in the Caucasus, near Tiflis, states that it occurred immediately after an ordinary hail-shower, and that he could see the successive showers marching over the country, and noticed that, between the last edge of the falling hail and the front edge of the falling ice-blocks, there was a distinct break, through which he could see the sun shining on the hills in the background. It was on this particular occasion that the best specimens of crystal-bespangled hailstones have been recorded and sketched, but others have been reported from Natal, and quite recently from Philadelphia, October 1, 1889. When such a visitation of ice-lumps takes place, the entire crops of the district affected by it are destroyed. Such a storm passed over Richmond in August, 1879, and in five minutes some ten thousand pounds' worth of damage was done, principally to conservatories. Naturally, Kew Gardens were among the principal sufferers.

It is a problem as yet unsolved to account for the suspension in the atmosphere of such objects as these hailstones, which frequently weigh much over an ounce. A recent theory, which seems to carry some probability with it, supposes that in the heart of every hail-cloud there is a whirlwind, or what is usually but erroneously termed a "tornado." It is well known that such disturbances exert a prodigious lifting power, raising heavy objects, such as carts, house-roofs, and even trees, and transporting them to considerable distances. The theory is, that when a drop of water in such a cloud is congealed it is carried round and round in the vortex and lifted up, more moisture being condensed and frozen upon it at each gyration, until at last it is thrown out and falls. This would account for the alternate layers of which I have already spoken, but will not account for the formation of crystals, a growth which usually requires a considerable time.