OBSERVATIONS ON INSECT LIFE 165 



generated by the contact of the air with the heated 

 rocks. That these currents rise with great force from 

 the valleys is indicated by the ease with which the 

 plumed seeds are swept aloft from between the cliffs 

 into the open sky, or the way in which a rain-cloud 

 pouring over a ridge is opposed by an ascending 

 stream within the valley and driven upwards as 

 though in wreaths of smoke. The mornings show 

 a clearer sky. The dust has settled in the cool of 

 night. But it is only a temporary lapse ; the haze 

 again deepens with the daily heat. Nothing but rain 

 can now purify the air. 



At last the clouds collect over the hills ; first in 

 silvery wisps increasing each evening into heavy 

 cumuli, and backing away at night, leaving a clear sky. 

 Finally the clouds burst ; a sense of relief is felt as 

 the rain pours down in torrents. The sky clears and 

 we see that the haze of dust has been swept to earth. 

 A vast panorama is now exposed ; thousands of 

 square miles of mountain are seen in a single view. 

 All that was wrapped in gloom appears through a 

 transparent sky. So clear is the prospect that every 

 object looks magnified as though drawn miles nearer 

 to our vision. Shadows cast by a passing cloud or by 

 a mountain side roll across the clear valleys. Sinuous 

 rivers sparkle in the sunlight ; tiny villages or strips 

 of cultivated soil, hiding in some narrow glen, for the 

 first time appear to view ; the nearer hills are tinged 

 with blue as if reflecting the azure of the sky, and 

 the snowy peaks climbing above the wooded slopes 

 contend with the whiteness of the clouds. 



Thus do the waves of heat, rising from the enclosed 

 valleys and lifting the finest particles of dust, darken 



