448 METEOEOLOGY. 



ing. It attains its liigbest elevation from two to three o'clock in the af- 

 ternoon, during the greatest heat, after which it fades away gradnally, 

 and a little after sun-down disappears entirely behind the hills. Thus 

 cumulus in Mexico is only visible in summer and during the day. How- 

 ard says : 



Clouds in tliis inodificatioii are coiumoaly of the most dense structure ; tlioy are 

 formed in the lower atmosphere and move along with the current which is nest the 

 earth. A small, irregular spot first appears, and is, as it were, the jn/cZcits on which 

 they increase. The lower surface continues iiTcgularly plane, while the upper rises 

 into conical or hemispherical heaps, which may afterward continue long nearly of the 

 same bulk or rapidly grow to the size of mountains. 



In the former case they are usually numerous and near together, in the latter few 

 and distant; but whether there are few or many, their hascs lie always nearly in one 

 horizontal plane ; and their increase upward is somewhat projiortiouate to the extent 

 of base, and nearly alike in many that ai>pear at once. 



Their appearance, increase, and disax^pearance in fair weather are often periodical 

 and keep pace with the temperature of the day. Thus they will begin to form some 

 hours after sunrise, arrive at their maximum in the hottest iiart of the afternoon, then 

 go on diminishing, and totally disperse about sunset. 



But in changeable weather they partake of the vicissitudes of the atmosphere; some- 

 times evaiJorating almost as soon as formed ; at others suddenly forming and as quickly 

 passing to the compound modifications. 



The cumulus affair weather has a moderate elevation and extent, and a well-defined, 

 rounded surface. Previous to rain it increases more rai^idly, ai>pears lower in the 

 atmosphere, and with its surface full of loose fleeces or x)rotuberances. 



The formation of large ciunulus to leeward in a strong wind indicates the approach 

 of a calm with rain. When they do not disappear or subside about sunset, but con- 

 tinue to rise, thunder is to bo expected in the night. 



Indexiendently of the beautj^ and magnificence it adds to the face of nature, the cumnlus 

 serves to screen the earth from the direct rays of the sun; by its multiplied reflections 

 to diftuse, and, as it were, economize the light, and also to convey the jiroduct of eva- 

 Xjoration to a distance from the place of its origin. The relations of the cumulus with 

 the state of the barometer, &c., have not yet been enough attended to. 



VII. — Pallio-cumulus, (Poey.) 



i^fliw-cZozff/, (illustration No. XII.) — Thejx<i/io-cj(WH?its is produced by the 

 accumulation of Jracto-cumulus, which is gradually extended under the 

 form of a uniform and compact stratum. This stratum is constantly sus- 

 tained by the entrance of new fracto-cunmli, which increase its thickness 

 until rain begins; then the/Vacto-cumwi^tts ceases to penetrate it, and passes 

 along the stratum of. imllio-cumulus^ but before the end of the rain it is dis- 

 engaged anew from the stratum, which grows thin, is broken up and dis- 

 appears. TliQ, indlio-cumulus is lower, more dense, less compact, more 

 rapid than the imllio-cirms^ and is slate-colored or grayish. The thicker 

 and more compact this stratum is, the longer will the rain continue; 

 but as soon as a breach is made, it disengages fragments of cumulus, 

 {fracto-cumulus,) which rapidly disappear, while the remainder are piled 

 up at the horizon in the form of cumulus. The pallio-cumulus appears 

 almost always from the northeast, showing the inferior polar current, 

 which soon reaches the surface of the earth. The meteorological mani- 

 festations determined by them are inverse to those of fiallio-cirrus ; the 



