THE HYDRAULICS OF GREAT RIVERS. 327 



in the same race, between those subject to a moist heat and those sub- 

 ject to a dry heat, for the purpose of suggesting its probable connec- 

 tion with the fact that the lighter-skinned races are habitually the 

 dominant races. We see it to have been so in Egypt. It was so with 

 the races spreading south from central Asia. There is evidence that 

 it was so in Central America and Peru. And if, heat being the same, 

 darkness of skin accompanies humidity of the air, while relative light- 

 ness of skin accompanies dryness of the air, then, in this habitual 

 predominance of the lighter-complexioned varieties of men, we find 

 further evidence that constitutional activity, and in so far social devel- 

 opment, is favored by a climate conducing to rapid evaporation. 



I do not mean that the energy thus resulting determines, of itself, 

 higher social development : this is neither implied deductively nor 

 shown inductively. But greater constitutional activity, making easy 

 the conquest of less active races and the usurpation of their richer and 

 more varied habitats, also makes possible a utilization of such habitats 

 that was not possible to the aborigines. 



THE HYDEAULICS OF GEEAT EIYEES. 



AX important advance in our knowledge of hydraulics has been 

 recently effected, through the observations of M. Eevy, a mem- 

 ber of the Institute of Civil Engineers of Vienna, on the great rivers 

 of Parana and Uruguay in South America. The results of his observa- 

 tions have just been published in England, in a book entitled *' The 

 Parana, the Uruguay, and the La Plata Estuaries," an excellent ac- 

 count of which appears in the April number of the Edinburgh Reziew^ 

 from which the following statement is derived. Before giving the de- 

 tails and results of M. Eevy's observations, it is advisable to furnish a 

 brief description of the character and appearance of the streams ob- 

 sarved. 



The La Plata is simply an estuary or arm of the sea into which 

 empty the Uruguay and the Parana. It trends in a northwesterly 

 direction, is 70 miles wide at its mouth, and 150 long to the mouth 

 of the Uruguay. Higher up still it loses itself in the Paraud Guayaza 

 and the Parana de las Palmas, embouchures of the Parana proper, 

 which branches 72 miles above to form a delta. The Uruguay and 

 the Parana are the main arteries of the vast basin formed by the 

 Andes on the west, the mountain-chain which runs parallel with 

 the Atlantic Ocean on the east, and on the north by the great range 

 of Cordilleras which stretch directly across the South American Con- 

 tinent at the fifteenth parallel of south latitude. Their water-shed 

 is the southern slope of these Cordilleras, while that of the Amazon 



