6i8 TEE POPULAR SCIENCE MONTHLY. 



native wells, give exit, from a mean depth of seventy metres, to a vol- 

 ume of water fully equal to that of the Seine, at Paris, in its lowest stage. 

 Cultivated lands have been created, the native population has doubled, 

 and the value of the oases has more than quintupled ; a complete 

 transformation of this part of the Sahara has been effected, by the 

 agency of underground waters, within thirty years. Most of the man- 

 ufacturing cities of the middle and north of England are situated upon 

 the New Red Sandstone, where, besides excellent building-stone and 

 proximity to the coal-fields, they enjoy the inestimable advantage of 

 tlie presence of inexhaustible reservoirs of water purified by natural 

 filtration, and easy of extraction. Belfast, in Ireland, is similarly situ- 

 ated. The water-bearing gravels are particularly worthy of attention 

 from this point of view. With the inexhaustible and easily accessible 

 provisions of water which they contain, they present to man an almost 

 infinite expansion. This accounts for the existence upon these de- 

 posits, from most ancient times, of numerous important cities and 

 capitals, like London, Paris, and Berlin. But in London the arenaceous 

 and phreatic stratum has limitations which were opposed for several 

 centuries to growth in particular directions. For a long time, accord- 

 ing to Mr. Pi-estwich's observations, the population, by an instinct easy 

 to understand, continued strictly concentrated on the principal water- 

 sheet, and on a few isolated strips of gravel, as at Islington and High- 

 bury. In the suburbs, likewise, the thick populations were collected 

 on the larger gravel-beds rich in water, while in the same region, al- 

 though the soil was everywhere cultivated and productive, the houses 

 were very sparsely scattered. But the situation has greatly changed 

 within the last seventy years, a supply of water having been brought 

 from a distance, and the city has spread very rapidly over the clayey 

 grounds. 



Kumcrous populations still depend on wells for their drinking-wa- 

 ter ; Lombardy and Venice, with two million inhabitants ; the exten- 

 sive plains of Hungary ; at least half of the German Empire ; a part of 

 the Russian Empire, seven times as large as France, and populated by 

 about twelve million souls ; and, according to the explorer Abbe David, 

 the whole of the great northern plain of the Chinese Empire, contain- 

 ing more than a hundred million inhabitants. Besides these vast plains 

 which represent more than a third of the continents, there are number- 

 less valleys, with water-bearing subsoils, which have attracted to them- 

 selves aggregations of men. We can then affirm that a very important 

 fraction of the human race depends for its principal drink wholly upon 

 water which is furnished by the phreatic strata of ancient or modern 

 alluviums. We never find such concentrations of inhabitants in coun- 

 tries where the soil is formed from granitic and schistose rocks, with- 

 out being covered by disaggregated materials. These rocks permit 

 water to descend to their interior only with the greatest difficulty. 

 Springs arc likewise weak among them, but very numerous ; and the 



