216 | PHYSICS. 
and 5000 feet. Pouillet instituted very exact measurements in 1840, 
during which he found clouds at an elevation of from 22,300 to 38,000 
feet. In general we may assume that the thin cirrus cloud does not descend 
below 2000 or 3000 feet of elevation, while the thicker rainy clouds may come 
within a few hundred feet of the earth, although they may occur at much 
greater elevations. Clouds appear, furthermore, to attain a greater height 
in low than in high latitudes, the watery vapor being carried higher in the 
former than in the latter. ) 
Jt is very difficult to ascertain the distance of a cloud from us, its 
apparent place being of not the slightest use in the determination. When 
the distance of a cloud is unknown, it becomes impossible to find out its - 
actual size; and even the very shape is sometimes a matter of ambiguity. 
A change in the position of the cloud causes a change in its external 
appearance. This is shown in pl. 24, fig. 11, where the observer at E sees 
the same cloud at one time lower under the angle AEC, at another time 
higher, and evidently very differently, under the angle BED. Thus the 
same cloud might appear quite dissimilar from two different stations at the 
saine instant of time. To an observer at A (fig. 10) the sky will appear 
furnished with clouds which are quite invisible to the one at B, the view 
being intercepted by a uniform stratum. 
Clouds of very different character are often brought into contact by aerial 
currents or changes in the density of the atmosphere. Thus in pl. 26, fig. 1, 
a cirrus and a cumulus cloud are apparently brought into contact; in 
jig. 2 a cirrus appears resting on the summit of a cumulus; in fig. 3 a 
cumulus appears to have its summit cut off by a horizontal layer of cloud. 
Such contacts of clouds are partly real, partly only apparent. The 
apparent occur when two clouds lie in the same line of sight from an 
observer, although they may actually be quite widely separated. Thus to 
the mountain observer at S (pl. 27, fig. 12) the two clouds, M and N, 
appear to be in contact, while to the one stationed on the plain at P, their 
relation to each other is very different. In pl. 26, fig. 4, also, it is only 
apparently that a range of cumulus appears embedded in a dense black 
layer of cloud. Fig. 5 shows two clouds actually in contact, but likewise 
apparently combined by a long thin streak of cloud. Fig. 6 presents an 
apparent mixture of cirrus with light transparent fog, relieved against a 
mackarel-back sky. Fig. shows an apparent contact of cirro-stratus with 
cirro-cumulus. 
Clouds under different illuminations present very different appearances. 
Sometimes a cloud appears entirely in shadow ; at another time its upper or 
lower border seems illuminated (pl. 26, figs. 8 and 9), A and B. As the 
shadows of clouds depend upon their different positions with reference to the 
sun, it is very evident that the same cloud will appear very differently in 
the morning and in the evening, in the north and in the south. If we 
assume that one mass of cloud, AB ( fig. 9), stands in the south, and another, 
A’B’, of similar shape in the north, then to an observer between them the 
northern cloud, A’B’, will present its illuminated face, exhibiting only a small 
portion of the shadow, while the other cloud will appear entirely in shade. 
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