SKY 15 



rora borealis; for with gas-lights and lamps 

 and electric glares in one's eyes, it is not 

 easy to see whether they are pulsing. 

 Only after midnight, from our yard, can I 

 be sure of this. 



These streams of cirrus cloud must be of 

 enormous length sometimes. You realize 

 it when you see their parallel lines drawn 

 together in each direction at the horizon, 

 like ridges on a muskmelon. But they are 

 not drawn together. They appear so to 

 us because they are in perspective, as the 

 sides of a street run together toward the 

 vanishing-point ; and as we can see a moun- 

 tain at a distance of a hundred miles in a 

 clear air, so in that clearer air above the 

 humid stratum we doubtless follow these 

 lines at least as far in each direction, or 

 two hundred miles in all. Occasionally a 

 cross wind scores these high clouds and 

 combs them into sections. Then, instead 

 of being streamers, they become endless 

 regiments marching in platoons in the 

 same direction as the original lines. 



Occasionally, too, the cirrus is so far and 

 thin that we do not see it in full day, 



