200 THE STORY-BOOK OF SCIENCE 



nutriment fails them. The emigrating band flies, as 

 at a given signal, and passes through the air in the 

 form of a great cloud that intercepts the daylight. 

 The migration continues for days at a time, so nu- 

 merous is the host. Then the voracious swarm 

 alights, like a living storm, on the vegetation of some 

 distant province. In a few hours grass, leaves of 

 trees, grain, prairies everything is browsed. The 

 soil, as if ravaged by fire, hasn't a blade of grass 

 left. Sometimes the people of Algeria die of hun- 

 ger. The grasshopper has devoured their harvests. 

 "Volcanoes cause cinder-showers. Volcanic ashes 

 is the name given to the calcined dust thrown up to 

 a great height by volcanoes at the moment of their 

 eruption. These powdered substances form enor- 

 mous clouds, which produce in the daytime a dark- 

 ness like that of the darkest nights, and which, falling 

 to earth at a greater or less distance, stifle animals 

 and plants under their showers of dust." 



