214 PHYSICS. 
few feet of diameter, and vanishing at a certain height of air. This 
generally occurs during rainy weather, and is usually a sign of its 
continuance. 3 
The densest fogs occur over cities; and these, in addition to vesicles of 
water, appear composed of various exhalations. This, for example, applies 
to the dense fogs of London, Paris. and Amsterdam, which may therefore 
be called mixed fogs. 
Besides the fog composed of vesicles of water, there is a dry fog, the 
principal compound of which is probably smoke or other exhalatory matter. 
Occasionally it occurs in great quantity, as in the years 1783 and 1847. 
Without diminishing the transparency of the air as much as is done by 
fogs, it dims the sun, and causes him to appear red and shorn of his beams, 
This phenomenon is most frequently observed in the Netherlands, in 
Northern France, and Germany ; more rarely in England, and still more so 
in Southern and Eastern Europe. In the United States it 1s especially 
prevalent, particularly in autumn, about the time of what is called Indian 
summer. Natural philosophers are not agreed as to the cause and 
components of this dry fog or haze; according to some it is of electrical 
origin, according to others it consists of the smoke and exhalations of 
burning and decomposing substances, as also from volcanoes. The latter 
theory is probably the correct one. The occurrence of this fog in Germany 
unquestionably depends on the burning of turf or moors in Westphalia and 
East Friesland, especially in the moor regions of the coast of the North Sea, 
where from May to July the turf is dug up, dried, and burned for manure. 
The spontaneous burning over of extensive tracts of forest land would 
also contribute to a very great extent. 
Of Clouds. 
Cloud is nothing more than fog which has reached or been formed in the 
higher regions of the atmosphere. Although the water of which the cloud 
is formed is heavier than air, yet its disposal in the shape of hollow spheres 
diminishes very much its gravity ; and even should it sink through, the lighter 
strata of the air, it will ultimately come to a denser, warmer, and less 
saturated part of the atmosphere. Here the cloud will be dissolved at its 
lower face, while new fog will be condensed on it from above. Hence the 
nearer the atmosphere is to the point of saturation, the lower do the clouds 
sweep. Whenever the cloud gets into a current of air it is carried along 
with it. 
The classification of clouds now almost universally adopted is that of 
Howard. He establishes three elementary and four secondary forms. The 
primary forms are: Ist. cirrus (pl. 25, fig. 1), consisting of light and 
feathery streaked filaments seen in clear weather ; 2d. cumulus (fig. 2), 
composed of huge hemispherical masses, apparently resting on a horizontal 
base, occurring chiefly in summer, and presenting the appearance of heaps 
of snow; 3d. stratus (fig. 3), an extended horizontal layer of cloud, 
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