78 



NATURE FOB ITS OWN SAKE 



Cloud illu- 

 sions. 



Gold and 

 silver Mn- 

 ing*. 



or across them. In outline they are graceful, and 

 in light-and-shade they are often sharp-marked. 

 The best time to study them is in the evening, 

 when they are lying back at the south or east. 

 Then the pinnacles and peaks glow with light, 

 and make the snowy-mountain illusion more 

 palpable than ever ; or they turn into phantom, 

 rock-based promontories with spectral tides of 

 vapor at their feet that sound not and shock 

 not, yet rise slowly higher and higher upon the 

 snowy walls. Occasionally a tall, heavy mass is 

 veiled by a thin layer of the stratus, through 

 which the form of the cumulus is seen to burn 

 like a great opal. Sometimes, too, a heavy 

 cumulus is seen through city smoke at sunset 

 glowing like molten metal. When in the west 

 and in front of the sun this cloud is the one 

 that shows us the gold or silver lining ; and 

 under sunset light it is possible for it to take 

 on all tints and shades. When it is not near 

 the sun but lies off at the side, we often see the 

 pink, " Alpine glow " suffusing the white cas- 

 tellated tops ; and the shadows caused by sharp 

 breaks of form often show blue, lilac, and even 

 pale green in hue. 



The cumulo-nimbus (a) is substantially the 

 same cloud as the cumulus except that it drifts 



