74 



NATURE FOR ITS OWN SAKE 



an altitude of thirty thousand feet, and has a 

 maximum travelling velocity of about seventy 

 miles an hour. It is the substance from which 

 the halos about the sun and moon are woven, 

 and is very thin, almost transparent. Like the 

 cirrus, it casts no patches of shadow, is pale 

 white, and when struck from beneath by the 

 rays of the sun below the horizon is marvellous 

 in its delicacy of light and color. 



The cirro-cumulus (b) is another mixed 

 cloud. When the cirrus descends still lower 

 than the region of the cirro-stratus, its edges 

 of frost begin to melt like the sharp sides of a 

 snow-bank. It then takes on a woolly appear- 

 ance similar at times to the small, detached 

 portions of true cumulus, though it lies in a 

 much higher field of air. It has a fashion of 

 breaking up into small, rounded patches like 

 rotten ice in a river, and of drifting across the 

 sky in vast companies that almost hide the 

 blue. There are two forms in which it ap- 

 pears. One is called the " dappled sky " or 

 sometimes " wool - pack " from its fleecy nat- 

 ure ; the other is the " mackerel sky," which 

 is not fleecy but hard-looking. The latter is 

 rarely seen as compared with other cloud forms, 

 and in England it is always thought to be the 



