224 PHYSICS. 
or the shower begins with single rain-drops followed by hail, the whole 
again concludjng with rain. This alternation sometimes occurs in May and 
June, rarely in July, and perhaps never in August and September. Not 
unfrequently sleety hail is mixed with snow, either at the beginning, or 
more frequently towards the end of winter, in February or March. Fine 
granular hail frequently occurs on high mountains, where regular hailstones 
seldom are seen ; thus on the higher Alps Saussure found twelve falls of the 
former to one of the latter. 
The real hail storms belong to the summer season, and are accompanied 
by the severest thunder and lightning. They are therefore most frequent 
in June and July, rarer in May, August, and September, and seldom 
experienced in April and October. Storms of this kind generally arise after 
clear calm weather, accompanied by long continued oppressive sultriness. 
The hail clouds appear to sweep very low, with their edges jagged, and 
their lower faces presenting irregular projections, the parts yielding hail 
generally forming remarkable white streaks, the rest of the cloud being very 
dark. The barometer and the thermometer sink rapidly, the latter some- 
times as much as 77° I*. A peculiar rustling in the air announces near and 
heavy hail clouds ; subsequently a darkness ensues not dissimilar to that 
produced by an eclipse of the sun. The hail itself lasts but a few minutes, 
with short interruptions, and generally accompanied by thunder and 
lightning, the duration being rarely over fifteen minutes, which, however, is 
often sufficient to cover the ground with a thick bed of ice. Hail storms 
move with great velocity ; the one which occurred in France in 1788, so 
remarkable for its great exient, traversed the country at the rate of 38.8 
Paris feet in a second, or forty geographical miles in an hour. The force 
with which the hail falls is very great, and, indeed, greater as the wind is 
more violent. Men are not seldom injured and even killed, a calamity 
which occurs quite frequently to small animals. Window-glass is very 
often broken, and even the roofs of houses and the branches of trees are at 
times unable to resist the terrible visitation. The injury produced in fields, 
gardens, &c., is often almost irreparable, as in the storm of 1788 in France, 
which devastated 1039 parishes, and caused a loss to the amount of more 
than twenty-five millions of francs. The quantity of hail which falls is 
rarely measurable with any degree of exactness, owing to the rapid melting 
produced by the accompanying rain, and the more elevated temperature of 
the air or ground. A depth of six inches is sometimes attained, although 
this very rarely occurs. at least in our latitude. A torrent of rain generally 
follows a fall of hail, lasting rarely more than half an hour. The area 
traversed by the hail is generally but small in width, often but a few 
hundred or thousand feet, rarely over a mile; the length of this area. again, 
is sometimes very great, amounting to over 400 miles in the storm just 
mentioned. 
In the tropics hail rarely occurs excepting at great elevations; in the far 
north, again, large hail is very seldom seen. In general the tract of hail 
proper is confined to the region between 30° and 60° degrees of latitude, 
and to elevations under 6000 feet. Even within these limits there are 
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