viii aiVEKS AND LAKES 333 



Some rivers, on the other hand, offer no 

 such periodical differences. The lower Rhone, 

 for instance, below the junction with the 

 SaQne, is nearly equal all through the year, 

 and yet we know that the upper portion is 

 greatly derived from the melting of the Swiss 

 snows. In this case, however, while the 

 Rhone itself is on this account highest in 

 summer and lowest in winter, the SaSne, on 

 the contrary, is swollen by the winter's rain, 

 and falls during the fine weather of summer. 

 Hence the two tend to counterbalance one 

 another. 



Periodical differences are of course com- 

 paratively easy to deal with. It is very dif- 

 ferent with floods due to irregular rainfall. 

 Here also, however, the mere quantity of rain 

 is by no means the only matter to be con- 

 sidered. For instance a heavy rain in the 

 watershed of the Seine, unless very prolonged, 

 causes less difference in the flow of the river, 

 say at Paris, than might at first have been 

 expected, because the height of the flood in 

 the nearer affluents has passed down the river 

 before that from the more distant streams has 



