100 0)1 the Modifications of Clouds, and 



cirrus spreads, while the cumulus enters it laterally and 

 troui beneath. 



Of the Cirrus. 



Clouds in this modification appear to have the least den- 

 sity, the greatest elevation, and the greatest variety of extent 

 and direction. They are the earliest appearance after serene 

 weather. They arc first indicated by a tew threads pen- 

 cilled, as it were, on the sky. These increase in length, 

 and new ones are in the mean time added laterally. Often 

 the first-formed threads serve as stems to support numerous 

 branches, which in their turn give rise to others. 



The increase is sometimes perfcctlv indeterminate, at 

 others it has a very decided direction. Thus the first few 

 threads being once formed, the remainder shall be propa- 

 gated either in one, two, or more directions laterally, or 

 obhquely upward or downward, the direction being often 

 the same in a great number of clouds visible at the same 

 time : for the oblique descending tufts shall appear to con- 

 verge towards a point in the horizon, and the long straight 

 streaks to meet in opposite points therein ; which is the op- 

 tical effect of parallel extension. 



Their duration is uncertain, varying from a few minutes 

 after the first appearance to an extent of many hours. It is 

 long when they appear alone and at great heights, and 

 .shorter when they are formed lower and in the viciaity of 

 other clouds. 



This modification, although in appearance almost mo- 

 ti'inless, is intimately connected with the variable motions 

 of the atmosphere. Considering that clouds of this kind 

 have long been deemed a prognostic of wind, it is extraor- 

 dinary that the nature of this connection should not have 

 been more studied, as the knowledge of it might have been 

 productive of useful results. 



In fair weather, with light variable breezes, the sky is 

 seldom quite clear of small groups of the oblique cirrus, 

 which frequently come on from the leeward, and the direc- 

 tion of their increase is to windward. Continued wet wea- 

 ther is attended with horizontal sheets of this cloud, which 

 subside quickly and pass to the cirro-stratus. 



Before storms they appear lower and denser, and usually 

 in the quarter opposite to that from which the storm arises. 

 St'.^dy high winds arc also preceded and attended by streaks 

 running quite across the sky in the direction they blow in. 



The relations of this modiiieation with the state of the 



baronaeterj, 



