18 OF CLOUDS. CHAP. l.6. 



cirrocumulus or cirrostratus: when it passes to 

 the latter, it descends lower in the atmosphere, 

 its fibres become denser and in general more 

 regularly horizontal, and it usually appears 

 subsiding, or altering its form. The figure of 

 the cirrostratus, like that of the cirrus, is very 

 various; sometimes it consists in dense longi- 

 tudinal streaks ; at others, it looks like shoals of 

 fish; sometimes the whole sky is so mottled 

 with it as to give the idea of the back of the 

 mackerel; this has been called the mackerelback 

 sky; frequently it appears like the grains of 

 polished wood, or is composed of fine fibres dis- 

 posed after the manner of the fibres of muscles, 

 which often intersect each other. I have seen 

 the cirrostratus assume the reticular form, like 

 the cirrus, from which it can then only be dis- 

 tinguished by its greater degree of density. 

 This cloud is sometimes spread out into a plane 

 horizontal sheet, more or less dense: this is the 

 form in which the halo generally appears. All 

 clouds are capable of becoming lighter or 

 darker, according to their relative position with 

 respect to the sun : the cirrostratus, however, is 

 remarkable for exhibiting a great variety of 

 beautiful colours, according to its variation in 

 density, to other peculiarities in its structure. 



