their Prodilction, Suspension, and Destruction. 105 



tOmtnencement of rain, while the lower atmosphere is yet 

 too dr\^ ; also during the approach of thunder storms : the 

 indistinct appearance of it is chiefly in the longer or shorter 

 intervals of showers of rain, snow, or hail. 



The cumulo-stratus chiefly afl'ects a mean state of the 

 atmosphere as to pressure and temperature ; but in this re- 

 spect, like the other modifications, it affords much room 

 for future observation. 



Of the Ni}7il2/s, or Cumulo-cirro-stratus.. 



Clouds in any one of the preceding modifications, at the 

 same degree of elevation, or in two or more of them, at 

 different elevations, may increase so as completely to ob- 

 scure the sky, and at times put on an appearance of density 

 which to the inexperienced observer indicates the speedy 

 commencement of rain. It is nevertheless extremely pro- 

 bable, as well from attentive observation as from a consi- 

 deration of the several modes of their production, that the 

 clouds while in any one of these states do not at any time 

 let fall rain. 



Before this effect takes place they have been uniformly 

 found to undergo a change, attended with appearances suf- 

 ficiently remarkable to constitute a distinct modificatiou. 

 These appearances, when the rain happens over our heads, 

 are but impnfeetly seen. We can then only obser\'e, be- 

 fore the arrival of the denser and lower clouds, or through 

 their interstices, that there exists at a greater altitude a 

 thin light veil, or at least a hazy lurbidness. When this 

 has considerably increased we see the lower clouds spread 

 themselves till they unite in all points and form one uni- 

 form sheet. The rain then connnences, and the lower 

 clouds, arriving fron) the windward, move under this sheet 

 and are successively lost in it. When the latter cease to 

 arrive, or when the sheet breaks, every one's experience 

 leaches him to expect an abatement or cessation of rain. 



But there often follows, what seems hitherto to have bcea 

 unnoticed, an immediate and great addition to the quantity 

 of cloud. At the same time the actual obscurity is lessened, 

 because the arrangement, which now returns, gives freer 

 passage to the rays of light: for on the cessation of rain 

 the lower broken clouds which remain rise into cumuli, 

 and the superior sheet puts on the various forms of the cirro- 

 stratus, sometimes passing" to the cirro-cumulus. 



If the interval be !on<r before the next shower, the cumulo- 

 diratuii usually makes Its appearance, which it also dues 

 sometimes verv suddenly after the first cessation, 



'But 



