xxvi RISE OF RIVERS IN SUMMER 141 



XXVI 



SOME rivers rise in summer like the Nile, of which i 

 I will give an account later on. Theophrastus 

 makes himself responsible for the statement that 

 in Pontus likewise certain rivers rise in the summer 

 season. Four different causes are assigned for this. 

 First, the earth is at that period most readily 

 changed into moisture. Second, there are in the 

 remote districts heavier rains, the water from which, 

 finding its way by secret channels, comes unnoticed 

 to swell the volume of the rivers. A third ex 

 planation is that the estuary is exposed to more 

 frequent winds, and is lashed by the sea waves ; the 

 river is checked and seems to increase because it 

 cannot discharge freely. The fourth reason connects 2 

 itself with the heavenly bodies. These bodies by 

 their more severe pressure during certain months 

 drain the rivers ; when they retire to a greater 

 distance, the waste and drain are less. What was 

 previously lost now accrues by way of increase. 

 Certain rivers fall visibly into some grotto or other, 

 and thus are withdrawn from sight ; some are 

 gradually wasted and disappear. They return, 

 however, at some distance off and recover their 

 name and course. The reason is plain enough. 3 

 There is vacant space underground. All liquid 

 naturally is carried to the lower level and to the 

 unoccupied space. The rivers received into these 

 recesses have run their course there in secret. But 

 as soon as any solid obstacle blocks the way, they 

 burst through the part that offers the slightest 

 obstruction to their escape and regain their channel 

 above ground. 



