METEOROLOGY. 215 
increasing from below, and appearing at times about sunset, of extraordinary 
brilliancy. The secondary or compound clouds are—Ist. cirro-cumulus 
( fig. 4), forming the transition from cirrus to cumulus, and constituting the 
aggregations of small round white clouds, resembling sheep in a meadow ; 
2d. cirro-stratus (fig. 5), consisting of cirrus combined in horizontal or 
slightly inclined layers of considerable extent ; 3d. cumulo-stratus ( fig. 6), 
often giving to the horizon a bluish-black color, frequently seen to great per- 
fection towards night of dry and windy winter weather ; 4th. nimbus (fig. 7), 
or rain cloud. fg. 8 exhibits cirrus and cumulus in the upper part above 
the thunder cloud. It must be readily understood that the precise reference 
of a cloud to one or the other of these modifications just described, must be 
sometimes a matter of great difficulty. 
The sheep-like cirro-cumulus, when verging closely on cirrus, generally 
precede clear mornings and evenings, and are mostly indicative of 
continued good weather. Many philosophers, Kaemtz among the number, 
suppose the cirrus cloud which soars far above the others, frequently at a 
height of 20,000 feet above the earth, to consist not of vapor, but of 
particles of snow or ice. The cumuli are most abundantly seen in the 
horizon, and are of dazzling whiteness on their borders, and dark in the 
middle. Their appearance is frequently not unlike a snow-capped 
mountain. The stratus is often nothing other than a layer of fog or mist, 
and occurs at all heights, frequently of great extent. Cirro-stratus occurs 
in various forms and colors, being generally seen in morning and evening, 
and giving rise to the beautiful redness of the sky. According to Howard, 
that haze covering an otherwise clear sky as with a veil, and most 
frequently the forerunner of bad weather, belongs to this class. The thick 
rain-threatening clouds arising from the combination and expansion of 
cumulus belong to the cumulo-stratus. 
The position of cloud is generally horizontal, it being only rarely that 
single clouds depend vertically, as in the case of wind and water spouts. 
The thickness of the different clouds is very various and difficult of deter- 
mination. Peytier and Hoffard ascertained the thickness of cloud strata in 
the Pyrenees to be 1400 and 2600 feet in two successive days. The 
greatest observed thickness amounts to 5000 feet, although cases must 
occur where this is vastly exceeded. 
The height of the clouds is much better known, although there are con- 
siderable difficulties in the way of ascertaining thiselevation. Various methods 
of different degrees of merit have been proposed by Riccioli (for two observers 
at a known distance apart), Wrede (making use of the shadows of clouds), 
Kaemtz, Arago, and others. More recently Wartmann has proposed the 
use of the artificial horizon. According to Riccioli, the maximum height 
of the clouds is 25,000 feet. According to the measurements of Lambert, 
the minimum height is 7300, the maximum from 15,000 to 20,000 feet. 
Gay Lussac, after ascending in his balloon to a height of 21,600 feet, saw 
small clouds still at a considerable distance above him. According to 
Kaemtz, cumulus sweeps along at a height of between 3000 and 10,000 
feet ; cirrus between 10,000 and 24,000 feet ; thunder-cloud between 1500 
389 
