14 NATURE IN A CITY YARD 



insignificant by contrast. Warmed air is 

 constantly rising from the earth, and as it 

 ascends toward the chill of the immensi- 

 ties, the moisture it holds condenses into 

 fog and occasionally into rain. The upper 

 edge of the bed of warm air defines its 

 shape by the form of the cloud-bottoms 

 that rest upon it. Floors of heavy cloud 

 average level; but there are innumerable 

 protuberances and depressions. At the 

 top, the air being thinner, the clouds ex- 

 pand into any shape they please. Away 

 up, miles overhead, where the air is too 

 light to contain or support masses like the 

 cumuli, the vapor feathers into cirri. The 

 cumuli, the summer clouds, which deepen 

 into thunder-heads, are Alpine in their 

 scenery and imposing in their volume ; 

 but there is something equally fine in the 

 cirrus when it is drawn into streams of 

 pallid white, like the banners flung from 

 the top of the world and blown by electric 

 currents into our heavens. Indeed, on 

 some nights, when the sky is charged with 

 cirri that faintly reflect the city lights, it is 

 hard to say whether or no they are the au- 



