Q SPRINGS, RIVERS, 6ANALS, LAKES, 



eruptions were not only frequent but for the most part very destruo 

 ' Thus in I he reign of Trajan, we are told, that it overflowed 

 j! ks iih prodigious violence, laid great part of the city under 

 . overturned many houses, and produced so much damage to 

 f 1 -" 9 adjoining f^lds as to occasion a severe local famine; and all 

 this, notwithstanding that the emperor had endeavoured to guard 

 against the evil, by canals for carrying off the surplus water in the 

 case of tin inundation of the Tiber, Aurelian pursued another plan, 

 si hd deepened its channel, while he enriched its banks with numerous 

 an.! extensive wharfs. Still, however, it occasionally produced the 

 samp public mischief, and in the reign of Valentiniau, overflowed to 

 stHi a degree that it laid all the lower parts of Rome under water, 

 and the inhabitants were obliged to save themselves upon the hills: 

 where the greater number of them would have peribhed of hunger 

 had not Claudius, prefect of the city, sent them a seasonable supply 

 of provisions in boats. It was Valenlinian who crowned the Tiber 

 with the celebrated bridge, which was at first called the bridge of 

 Graiian, and afterwards of Cestus. It is the Ponto di S. Bai tolomeo, 

 or St. Bartholomew's bridge, of the present day. 



SECTION IV. 



PERIODICAL SPRINGS AND LAKES. 

 I. Introductoty 01 serrations. 



AMONG the natural phenomena that the surface of the earth dis- 

 plays to us, there are few more curious than those of intermitting 

 or reciprocating fountains or other beds of water; nor is it by any 

 means an easy matter to account for so extraordinary a fact. An 

 irregularity of flow is indeed by no means uncommon; most of the 

 "boiling springs are subject to it. But there are others that evince 

 almost as regular and periodical influx and reflux as the tides of the 

 ocean ; while, not unfrequently, these alterations occur several 

 times in a day or even in an hour. Perhaps the causes are various, 

 and are sometimes subterranean and at others superficial. Generally 

 speaking, springs and lakes ot this description have been ascertained 

 to communicate with a lower layer of the same, through pores or 

 apertures of various diameter, which serve equally to carry off tlie 



