u 4 PHYSICAL SCIENCE BK. m 



16 Besides, subtlety of thought practised on the 

 hidden mysteries of nature will prove no less 

 efficacious in problems that lie more on the surface. 

 And nothing is more on the surface than these 

 salutary lessons we are taught as safeguards against 

 the prevailing vice and madness faults we all con- 

 demn, but do not abandon. 



1 LET us enter then on an investigation of forms 

 of water, and let us trace the causes that produce 

 them ; whether, as Ovid says : 



There was a fountain silvery clear with gleaming wavelets ; 



or, as Virgil says : 



Whence through nine mouths with mighty roar of the mountain 

 The sea issues in broken waves, overspreading the fields with its 

 resounding flood ; 



or, as I find it in your own poem, my dear Lucilius : 

 The stream of Elis wells up from Sicilian fountains. 



Let us inquire by what method the waters are 



2 supplied ; how it is that day and night unceasingly 

 so many huge rivers roll down their course ; why 

 some are swollen by the rain of winter, some 

 increase in summer when all the other streams fail. 

 Meantime let us separate the Nile from the common 

 crowd ; it is a river of peculiar and unique character. 

 We shall give it its turn by and by. At present we 

 will confine our treatment to the common waters, 

 cold as well as hot. In regard to the latter we 

 must inquire whether the heat is due to natural or 

 artificial causes. We shall discuss other waters 

 too which are rendered remarkable by taste or some 



