44 OF CLOUDS. CHAP. 2. 1. 



sional appearance of cirri of fibrous texture 

 above, at a time when the hygrometer indicates 

 a humid atmosphere below. 



The plumose cirrus often appears when the 

 sky is deep blue, and the cirrus of fibrous struc- 

 ture sometimes appears when it is pale coloured. 

 But the intensity of the blue of the sky does 

 not seem to depend on the dryness of the air; 

 nor the paleness, on its moisture. In the inter- 

 vals of showers the intensity of the blue is 

 often the greatest.* While I am now writing, I 

 observe out of my window abundance of fibrous 

 cirri in a sky rather pale than otherwise. During 

 the abundance of cirri I have sometimes per- 

 ceived the sky particularly pale; which, on 

 minute examination, have been found to be 

 caused by innumerable fine fibres of cirrus lying 

 very close together. 



There is a variety of the cirrus, called in 

 Lincolnshire the Sea Tree, which has somewhat 

 of the plumose structure, and generally precedes 

 Rain. Its figure gives a faint resemblance to 

 that of a tree, whence it derives its name, one 

 end being a compact kind of trunk, from which 



* Sir Isaac Newton somewhere observes that the deepest 

 blue sky happens just at the change from a dry to a moist 

 atmosphere. 



