HAIL.] 



METEOROLOGY. 



1145 



Coloured Sivnv. Red and preen snow have been fre- 

 quently described by travellers. The cause of these 

 phenomena is now referred to the presence of minute 

 algje the protococcus nivulis in the so-called red snow, 

 and the protococcitt viridis in the green variety. 



HAIL. Frequently, cloud vesicles become aggregated 

 and frozen into lumps of various sizes and shapes, some- 

 times opaque, sometimes transparent, and occasionally, 

 though not very often, containing ueuclei of solid foreign 

 matters. Meteorologically, aggregations of this kind 

 constitute hail. In most parts of the world where rain 

 occurs, hail is known ; but certain localities are particu- 

 larly subject to hail-storms. Generally hail falls by day ; 

 indeed an opinion prevails that hail-storms are unknown 

 by night. This supposition is, however, erroneous. The 

 form of hail-stones is various, though for the most part 

 they assume a spherical, spheroidal, paraboloidal, or 

 jiyriform contour ; and still more frequently they are 

 rounded, flattened, or angular. According to Delcross, 

 the most common form of hail is that of a three-sided 

 spherical segment, resulting from the comminution of 



r spheres. 



The diameter of hail-stones at a mean latitude is, 

 according to Muncke, not usually greater than one and 

 a-half, or one and three-fourths of an inch, although, on 

 (tome occasions, blocks of ice of enormous dimensions 

 have fallen. For example, in 1719, there fell at Kremo, 

 hail stones weighing not less than six pounds ; and at 



iiir, in 1717, others weighing not less than eight 

 ] iind.i. Again, it is stated that, in 1680, masses of ice 



in the Orkneys, twelve inches thick; and in 1795, 

 hail stones fell in New Holland, from six to eight inches 

 long, and two fingers thick. It is recorded, but on 

 doubtful testimony, that there fell in Hungary, on May 

 28th, 1802, a piece of ice three feet square by two feet 

 thick, and the weight of which was 1,100 pounds. But 

 even this is much exceeded by a statement that, in the 

 latter part of the reign of Tippoo Saib, a lump of ice fell 

 at Seringapatam as large as an elephant. The size of the 

 elephant, however, is not mentioned. 



With regard to foreign substances existing in hail- 

 atones, they are described as various. In 1755, there 

 fell, in Iceland, hail-stones contaitiing sand and volcanic 

 ashes ; others whu-h fell in Ireland, in 1821, contained a 

 metallic nucleus, which proved, on analysis, to be iron 

 pyrites (sulphide of iron) ; a similar phenomenon occurred 

 in Siberia in the year 1824. The presence of small 

 pieces of straw in hail has been frequently demonstrated. 

 The largest hail-stones fall in summer, during thunder- 

 storms. Storms of this kind are most frequent in June 

 and July ; they are more rare in May, August, and 



in bur, and still more so in April aud October. 

 Thi.-y usually occur at the close of long periods of calm, 

 sultry weather. Hail-clouds are much lower in the sky 

 than min clouds ; and are generally recognisable by a 

 peculiar ragged or jagged contour, and by their lower 

 portions being marked with white streaks, the other 

 portions of the cloud being inky black. Previous to the 

 occurrence of hail, the barometer sinks very low ; and, 

 what is unusual before rain, the thermometric column 

 suiters a corresponding depression. The thermometer, 

 during a hail-storm, has even been known to sink through 

 77 !'. A peculiar rustling sound in the air is also 

 indicative of speedy hail, accompanied by a darkness 

 resembling that dependent on an eclipse of the sun. 

 Hail-storms are very seldom of long duration, usually 

 lasting a few moments only seldom longer than a 

 quarter of an hour. The rapidity with which hail-storms 

 travel is very great : one which occurred in Central 

 France, in 1788, travelled at the rate of forty miles an 

 hour. The force of hail stones is sometimes dangerously 

 great, not only breaking windows and shattering tiles, 

 but killing men and animals, cutting oil branches of trees 

 and herbage, and, in short, desolating all save the largest 

 vegetable growths. The bail-storm in France of 1788, 

 already adverted to, extended its devastations over l,o:;;i 

 parishes, destroying property to the amount of 25,000,000 

 of francs. Although hail-storms often extend very far 

 in a linear direction, their breadth is usually iucou- 

 TOL. L 



siderable often but a few hundred, or at most a few 

 thousand feet ; while the linear extension has beeii 

 known to exceed four hundred miles. 



It has been already mentioned, that wherever rain- 

 clouds rest, hail may occur ; nevertheless, latitude and 

 local conditions determine the frequency of the pheno- 

 menon. Rain seldom occurs on the level land of tropical 

 countries ; and it is rare in the extreme north. The 

 hail-belt, pre-eminently so considered, is comprehended 

 between 30 and 60, aud to elevations less than 6,000 

 feet. Even within this belt, and below the limit of 

 elevation just assigned, there are certain localities where 

 the occurrence of hail is a very rare phenomenon. 

 Certain of the Swiss valleys maybe cited as a well-known 

 illustration of this remark ; more especially in the Valais 

 and its allied dales. It has also been well determined 

 that hail more rarely occurs at the base of mountains 

 than in localities a short distance removed. Perhaps no 

 country, upon the whole, is more subject than France to 

 the ravages of hail-storms, and in no country are the 

 effects more serious. It has been ascertained that the 

 average annual number of hail-storms in France is about 

 fifteen. They are especially prejudicial to the vine and 

 the olive, sometimes laying whole districts under desola- 

 tion. Having regard to the highly excited electrical 

 condition of the atmosphere, as the rule, during the 

 occurrence of hail storms, great hopes were once enter- 

 tained that they might be prevented, or their ravaging 

 power diminished, by means of suspended conductors. 

 The idea of using such conductors appears to have been 

 first suggested by Guenaut de Montbeillard, in 1776 ; and 

 hail-conductors have been extensively tried, but hitherto 

 without any amount of practical benefit to justify their 

 longer continuance. In 1820, a peculiar kind of hail- 

 preventer was suggested by La Postolle, and subse- 

 quently by Thollard. The instruments consisted of 

 straw ropes, in which a metallic wire was interwoven, and 

 suspended by means of pointed rods similar to lightning- 

 conductors ; but, like instruments having a similar 

 object, and which preceded them, they were found to be 

 unavailing. 



METHODS or DETERMINING THE AMOUNT OF ATMO- 

 SPHERIC MOISTURE. Having described the numerous 

 forms under which aqueous moisture may exist iu the 

 atmosphere, it now remains to indicate and describe 

 certain instruments which have been devised by different 

 experimenters for determining its amount. These 

 instruments, founded on different principles, as will be 

 seen, are termed hygrometers. 



It is a matter of common experience that many bodies 

 are affected as regards their dimensions, more particularly 

 their linear dimensions, by mutations of atmospheric 

 moisture. Wood is a very common example of this 

 property ; more particularly a stick of wood cut trans- 

 versely to the grain. Founded on this property of wood, 

 the late Mr. Edgeworth constructed a very ingenious 

 toy, which, though not a hygrometer, inasmuch as it does 

 not measure the amount of atmosphere prevalent in the 

 air, is at any rate a hyyroscope. It is related, that the 

 somewhat eccentric philosopher, just named, once laid a 

 wager that a certain toy a wooden horse constructed 

 by himself, should, after the lapse of some time, walk 

 across his room. The horse was accordingly made, and 

 placed at one end of a chamber ; the door of the chamber 

 wan then locked, and the key deposited in safe keeping. 

 In process of time the horse did indeed arrive at the 

 other end of the room ; and the manner in which this 

 was accomplished will now be made evident. 



Underneath each hoof was a claw, long enough to stick 

 into the flooring, and there take hold. The horse itself 

 was made out of a piece of wood cut transversely to the 

 grain ; the consequence was, that when the weather was 

 dry the linear dimensions of his back contracted, ami 

 when the weather was wet his back again elongated. 

 Now, bearing in mind the construction of the feet of this 

 toy-horse, it is evident that these alternate contractions 

 and expansions must necessarily result in a forward 

 motion. 



Again, the condition of human hair illustrates the 



7o 



