METEOROLOGY. 226 
countries where hail seldom or never occurs, as in some valleys of Switzer- 
land, especially in the Valais, and in most of the valleys extending east and 
west. In the low lands at the foot of high mountains, hail is more abundant 
at a certain distance from the mountain than nearer or more remote. The 
same region is sometimes ravaged by hail storms for several years in 
succession, and afterwards again spared for a considerable time. From the 
comparisons of Kaemtz, who assumes but one kind of hail, there occur in 
France every year from ten to twenty hail storms (the most in spring, the 
fewest in summer), over five in Germany, five in Rome, and three in the 
interior of Europe. 
Of the many propositions for preventing hail by the use of hail conductors 
none have yielded any practical results. The methods suggested may 
exert their influence in three ways: either by drawing off the electricity, 
by mechanical agitation of the strata, or by a chemically decomposing 
influence on the composition of the atmosphere. Hail conductors, or rods 
of the first kind, were proposed by Guenaut de Montbeillard in 1776. In 
1820 La Postolle, and after him Thollard, recommended hail conductors of 
straw ropes, attached to pointed rods, or of straw ropes with a metallic wire 
interwoven. Curiously enough, these methods were much followed without 
the least benefit flowing from them. At the present day most meteorologists 
are agreed that there is no certain, or at least practical method for 
preventing the occurrence of hail storms. 
6. On the Optical Phenomena of the Atmosphere. 
The air, although among the most transparent bodies in nature, is not 
perfectly so. Were this the case, its individual particles would reflect or 
scatter no light, and even by day the canopy of heaven would appear 
perfectly dark or black, with the exception of the space occupied by the sun 
and stars: sudden and total darkness would likewise ensue immediately on 
the setting of the sun. The general illumination of the heavens, and of 
objects not immediately reached by the direct solar rays, as also the gradual 
transition from daylight to the darkness of night, can only be explained on 
the supposition that the air does not transmit all the light, but reflects one 
part of the light traversing it, and absorbs another. The latter circumstance 
produces an enfeebling of the light, as we may readily perceive on examining 
remote terrestrial objects. In such an examination we find objects becoming 
more indistinct with increasing distance, vanishing at last altogether. The 
diminution of the angle of vision has of course something to do with this 
result, but not everything, since the effect thus produced varies at different 
times. 
An instrument called the diaphanometer, invented by Saussure, is used to 
determine the transparency of the atmosphere. This savant painted black 
circles of different diameters on a white ground, and then removing from 
them, determined the distance at which they disappeared ; this served as a 
measure of the transparency in question. In the higher regions of the 
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