CLOUDS. 447 



palUo-cirrus, it disappears entirely if fine weather is maintained. Let us 

 now see wliat are the inherent characters of the Uyo])alUo. 



V. — Pallio-cireus, (PoiiiY.) 



^7icef-cZoi(cZ, (illustration No. IX,) — Thei^aZKo-cirrMs is formed by the accu- 

 mulation of a cirro-cumulus, which is visibly sinking, or appears already 

 formed toward a point of the horizon in the stratum, corresponding to this 

 latter type. In the first case it is a little lower, more dense, less compact, 

 more rapid in its movement, grayish, and often shows some traces of polari- 

 zation. In the second case it is a little higher, less dense, more compact, 

 less rapid, i)early whitish, impenetrable to the solar rays, and without a 

 trace of polarization. In the two cases it appears generally in the south- 

 westerly horizon, showing the presence of the superior equatorial cur- 

 rent, and determining the fall of rain while it remains above and oppo- 

 site the 2Killio-cumulus. When a breach is made in this inferior stratum 

 it is speedily produced in that of the pallio-cirrus ; sometimes it has 

 already been formed in the latter. After the rupture of this stratum, 

 the jjalUo-cirrtis is transformed into cirro-cumulus, studded with cirro- 

 straUis. On the approach of pallio-cirrus we observe tlie following me- 

 teorological manifestations : the barometer falls, the thermometer rises, 

 the relative humidity increases, the tension of vapor diminishes, and 

 a little after the wind at the earth is felt from that direction. 



VI.— CUIMULUS, (vel CUMULO-STRATUS,) (HOWARD.) 



31ount-clou(l, (illustrations Nos. X andXI.) — Howard's cumulu s, [or cumu- 

 lo-stratus,) summer and aqueous vesicle clouds, (the " cotton-balls" of sail- 

 ors,) always appear in the form of a hemisphere or arcs of a circle, and repose 

 upon a horizontal base. When these hemispheres are piled upon one an- 

 other, there are formed great clouds accumulated atthe horizon, similar to 

 mountains in the distance covered with snow. Their contours take many 

 forms, human, animal, and of every kind, more or less hizarre and fan- 

 tastic, which inspired the poet Ossian with his finest images, and have 

 given rise in mountainous countries to many popular traditions. 



W^hen the cumuUis moves along the horizon, it is, excepting fracto- 

 cumulus, the most rapid of all clouds. But when it is piled up along 

 the horizon, in summer toward the south, in winter toward the north, 

 it is excessively slow, and remains a whole day scarcely moving. It 

 then extends perpendicularly or obliquely toward the zenith. Its 

 roundish summit is of a glittering whiteness, and when elevated high 

 enough, it is dyed rose color morning and evening like the cirrus. The 

 center of the cloud is grayish, the base slate color or black. The base 

 of the cumulus always rests upon the horizon, and is little elevated dur- 

 ing a storm. It follows the direction of surface winds. Upon the pla- 

 teau of the valley of Mexico, the cumulus completely disappears during 

 the winter half of the year, and appears anew during the other six 

 months. In summer it appears about eight or nine o'clock in the morn- 



