INUNDATIONS AND TORRENTS. 473 



the action of the destructive causes in question can be arrested or 

 perhaps even sensibly mitigated by their influence ; and besides, 

 floods will always occur in years of excessive precipitation, 

 whether the surface of the soil be generally cleared or generaUy 

 wooded.* 



* All the arrangements of rural husbandry, and we might say of civilized 

 occupancy of the earth, are such as necessarily to increase the danger and the 

 range of floods by promoting the rapid discharge of the waters of precipita- 

 tion. Superficial, if not subterranean, drainage is a necessary condition of all 

 agriculture. There is no field which has not some artificial disposition for 

 this purpose, and even the furrows of ploughed land, if the surface is in- 

 clined, and especially when it is frozen, serve rather to carry off than to retain 

 water. As Becquerel has observed, common road and railway ditches are 

 among the most efficient conduits for the discharge of surface-water which 

 man has yet constructed, and of course they are powerful agents in causing 

 river inundations. All these channels are, indeed, necessary for the conven- 

 ience of man, but this convenience, like every other interference with the 

 order of nature, must often be purchased at a heavy cost. 



It has long been supposed that large towns, great aggregations of men living 

 together under the artificial conditions of civic life, must exercise an appre 

 ciable influence on climate. But such influence has hardly yet been shown by 

 scientific observation. It is, however, at least certain that they intercept and 

 divert fronj their proximate, if not from their ultimate, points of delivery 

 into the common receptacle of the ocean, a large amount of precipitation. 

 London, for example, after making a sufficient allowance for parks, gardens 

 and other open spaces, has, within the jurisdiction of the Board of Works, at 

 least 100 square miles of substantially impervious paved or roofed surfaces. 

 Calculating the yearly rainfall at London at 24 inches, this city receives an- 

 nually a quantity of water of precipitation about equal to the mean delivery 

 of the Nile for forty minutes — a quantity not absolutely large, certainly, but 

 relatively great enough to be attended with important geographical conse- 

 quences ; for though, as is above admitted, this water may, at least in part, 

 finally reach the ocean, yet its channels, its rate of flow, and the amount of 

 evaporation from it, must be very different from what they would have been 

 had it fallen on an open tract of equal extent. Then again, this vast arti- 

 ficially covered space may produce important meteorological effects. When we 

 consider the heat of the domestic fires and lights required for the comfort and 

 convenience of so large a population, the forges and furnaces used for me- 

 chanical operations, the heat thrown off by the friction of machinery, of car- 

 riages and of the iron-shod hoofs of horses, the animal warmth disengaged by 

 men and beasts, the solar heat absorbed, accumulated, and given off through 

 radiation, by walls and other artificial constructions which greatly increase 

 the area of exposed surface, — when we consider all these things, it would 

 seem that a large city must be a source of heat potent enough to disperse 

 overhanging clouds and to dissipate much of the aqueous vapor that composes 

 them. 



