Reference to the Supply of Large Towns. 311 



they begin to be displaced and their foul presence replaced 

 by pure and healthy air they are at once apparent. 



Thus we can see that when God made the great rivers, 

 he rolled them down from the mountain to the sea over 

 rough beds that every atom of their contents could be ex- 

 posed to and filled with air. The brook first foaming along 

 its bed breathes the free air and carries a full burden of it 

 into the river, which constantly spreads its ever changing 

 surface out to the same exposure. And on the ocean 

 winds play over the water so as to insinuate into it as full 

 a supply of air as it can retain. A careful analysis with 

 reference to this .action, of the molecular motion which 

 produces waves, is remarkable in showing that were all of 

 the operations of undulation made subservient to this pur- 

 pose, they would not be better adapted to it. 



We shall find that besides the saline impurities which 

 water receives in its way to the sea from the clouds, there 

 are certain other conditions which may result from the 

 incomplete operation of causes already named. We can 

 see that the water which fell condensed from the air has 

 been exposed in its whole passage back to the sea, to the 

 germs of organic life, .and if they be developed, to the 

 organic impurities of their decay. All the changes which 

 have taken place have tended to prevent this. Salts 

 which were dissolved if in sufficient amount, and the 

 aeration kept up by constant motion have this tendency, 

 and when it reaches the sea it is subject to the increased 

 saltness and the action of the wind. 



Should there be a defect in any of these operations other 

 results follow. Now defects do not occur in nature, but 

 nature it seems never made any provision for large con- 

 gregations of men to live together. This we find in many 

 things, and in none is it more apparent than in the difficulty 

 which is met with in furnishing the natural requirements 

 for so many, though they be so bountifully supplied to all 

 who live apart. 



