190 LIGHT SCIENCE FOR LEISURE HOURS. 



boldt, by many kinds of birds, which suffer no injury 

 whatever, even when they skim the very surface of the 

 water. Yet there can be little doubt that Avernus 

 hides the outlet of an extinct volcano ; and long after 

 this volcano had become inactive, the lake which con- 

 cealed its site " may have deserved the appellation of 

 6 atri janua Ditis,' emitting, perhaps, gases as destruc- 

 tive of animal life as those suffocating vapors given 

 out by Lake Quilotoa, in Quito, in 1797, by which 

 whole herds of cattle were killed on its shores, or as 

 those deleterious emanations which annihilated all the 

 cattle in the island of Lancerote, one of the Canaries, 

 in 1730." 



"While Ischia was in full activity, not only was 

 Vesuvius quiescent, but even Etna seemed to be 

 gradually expiring, so that Seneca ranks this volcano 

 among the number of nearly-extinguished craters. At 

 a later epoch, ^Elian asserted that the mountain itself 

 was sinking, so that seamen lost sight of the summit at 

 a less distance across the seas than of old. Yet within 

 the last two hundred years there have been eruptions 

 from Etna rivalling, if not surpassing, in intensity the 

 convulsions recorded by ancient historians. 



We shall not here attempt to show that Vesuvius 

 and Etna belong to the same volcanic sytem, though 

 there is reason not only for supposing this to be the 

 case, but for the belief that all the subterranean regions 

 whose effects have been shown from time to time over 



