128 PHYSICAL SCIENCE BK. in 



XVI 



1 ANOTHER peculiarity requires explanation : some 

 wells are full for six hours and dry for six alter- 

 nately : why is this so? It is hardly necessary to 

 name the rivers individually which are at certain 

 months broad, at certain narrow, and to give 

 separate causes of this, seeing I can give a common 

 explanation that applies to all. An ague returns 

 at the same hour, gout always keeps its appointment, 

 the custom of women, unless interrupted, observes 

 its stated period, birth is ready at the proper month. 

 In like manner waters have their intervals of recur- 

 rence, at which to withdraw and at which to return. 



2 Now, some intervals are shorter, and the more strik- 

 ing on that account ; some are longer, but no less 

 certain. And what is strange in that, when you see 

 that the succession of events, and all nature, by decree 

 preserve their appointed order ? Winter has never 

 mistaken its time. Summer has always blazed forth 

 in its season. The changes of spring and autumn 

 have occurred according to their wont. Solstice and 

 equinox alike have kept their appointed days. 



Beneath the earth likewise there are laws of 

 nature, less familiar to us, but no less fixed. Be 

 assured that there exists below everything that you 

 see above. There, too, there are antres vast, im- 

 mense recesses, and vacant spaces, with mountains 



3 overhanging on either hand. There are yawning 

 gulfs stretching down into the abyss, which have 

 often swallowed up cities that have fallen into them, 

 and have buried in their depths their mighty ruins. 

 These retreats are filled with air, for nowhere is 

 there a vacuum in nature ; through their ample 



