CLOUDS. 449 



barometer rises, the thermometer falls, the relative humidity dimin- 

 ishes, aud the tension of aqueous vaijor increases. 



VIII. — Feacto-cumulus, (Poet.) 



Trm<?-cZow(7, (illustrations Kos. XIII and XIY.) — The clouds which I have 

 denominated/mc^o-CMmwZt(s are isolated fragments of ctimulus, more or less 

 considerable,without determinate form,jagged at the edges, the lowest and 

 most rapid of all, and whitish, grayish, or slate-colored, according to their 

 density. As soon as an invisil)le storm has broken out in the distance, we 

 see them moving with great rapidity, almost grazing the highest buildings 

 and tallest trees ; their borders are greatly torn and white, contrasting 

 strongly with the grayish stratum of superior palUocuniuJiis. They are 

 visible day and night, and often traverse the firmament from northeast 

 to southwest without interruption for many days, while the sky above 

 and in the intermediate space is perfectly clear. In winter we see them 

 alone under a blue sky, sending in their passage to the zenith inter- 

 rupted showers of rain, accompanied by strong gusts of wind, which 

 occasion a very slight elevation and oscillation in the barometric column. 

 In the Antilles these clouds produce the disagreeable winter rains, aud 

 in Europe the March storms. They generally follow the direction of 

 the wind i)revailing at the surface of the earth. When this wind is 

 contrary to the direction of fracto-cumulus, it soon takes the same course. 



A little before a storm or tempest arises, there appear a series of very 

 small fractO'CumuU, which move rapidly, almost to two-thirds of their 

 height, along a considerable mass of cumulus^ which is stationed very 

 often, as if immobile, near the southern horizon. Soon these fracto- 

 cumuli become more abundant, less rapid, and form a horizontal band, 

 which cuts the cumuli near their summit. This ajipearauce is a warning 

 sign for sailors, as it announces a squall. In fact, the fracto-cumuli 

 become more and more developed; an exchange of opposite electricities 

 takes i)lace between them, and the storm quickly arises. It is, there- 

 fore, the same little cloud, of which I have spoken above, that, returning 

 then from the combat, comes now to offer new battle. The aggregation 

 of fracto-cwniulus forms the ;paUio-cumulus, and increases and constantly 

 maintains this stratum. 



:NATUEE of the clouds deduced FEOM THE FOEMA- 

 TION OF HALOS, COEOXAS, AND EAINBOWS. 



We can further distinguish the nature of clouds by the optical pheno- 

 mena to which they give rise, according as their intimate constitution 

 is more or less connected with a certain degree of aqueous vapor, in the 

 state of snowy or glacial congelations of the strata corresponding to the 

 formation of each tyjie. The following are some facts which I have ob- 

 served at Havana, which it is important to verify in other regions : 



Generally speaking, cirrus, more esi^ecisilly pallio-cirrus, gives rise to 

 a large solar and lunar halo of 22c> radius. When it is produced by the 



