172 LIGHT SCIENCE FOR LEISURE HOURS. 



until it began to be evident that the volcanic fires were 

 about to return to their main vent. The obstruction 

 which had so long impeded the exit of the confined 

 matter was not, however, readily removed, and it was 

 only in August in the year 79, after numerous and 

 violent internal throes, that the superincumbent mass 

 was at length hurled forth. Eocks and cinders, lava, 

 sand, and scoriae, were propelled from the crater, and 

 spread many miles on every side of Vesuvius. 



We have an interesting account of the great eruption 

 which followed in a letter from the younger Pliny to 

 the younger Tacitus. The latter had asked for an 

 account of the death of the elder Pliny, who lost 

 his life in his eagerness to obtain a near view of the 

 dreadful phenomenon. 'He was at that time,' says 

 his nephew, ' with the fleet under his command at 

 Misenum. On August 24, about one in the afternoon, 

 my mother desired him to observe a cloud of very 

 extraordinary size and shape. He had just returned 

 from taking the benefit of the sun, and, after bathing 

 himself in cold water, and taking a slight repast, had 

 retired to his study. He arose at once, and went out 

 upon a height whence he might more distinctly view 

 this strange phenomenon. It was not at this distance 

 discernible from what mountain the cloud issued, but 

 it was found afterwards that it came from Vesuvius. 

 I cannot give a more exact description of its figure 

 than by comparing it to that of a pine-tree, for it shot 

 up to a great height in the form of a trunk, which 

 extended itself at the top into a sort of branches ; 



