360 - DYNAMICAL GEOLOGY.  — [Book Til. © 
possible for a generally applicable calculation, every area having its 
own peculiarities of underground drainage. The river Wandle, for 
instance, drains an area of 51 square miles of the Chalk Downs in 
the south-east of England. For eighteen months, from May 1858 
to October 1859, as tested by gauging, there was very little 
absorption of rainfall over the drainage basin, and yet the minimum 
recorded flow of the Wandle was 10,000,000 gallons a day, which 
represents not more than °4090 inch of rain absorbed on the 51 
square miles of chalk. The rock is so saturated that it can continue 
to supply a large yield of water for eighteen months after it {has 

ceased to receive supplies from the surface, or at least has received 
only very much diminished supplies.’ 
II. Discharge.—What proportion of the total rainfall is dis- 
charged by rivers is another question of great geological and 
industrial interest. From the very moment that water takes visible 
form, as mist, cloud, dew, rain, snow, or hail, it is subject to 
evaporation. When it reaches the ground, or flows off into brooks, 
rivers, lakes, or the sea, it undergoes continual diminution from the 
same cause. Hence in regions where rivers receive no tributaries, 
they grow smaller in volume as they move onward, till in dry hot 
climates they even disappear. Apart from temperature, the amount — 
of evaporation is largely regulated by the nature of the surface from. 
which it takes place, one soil or rock differing from another, and all 
of them probably from a surface of water. Full and detailed 
observations are still wanting for determining the relation of 
evaporation to rainfall and river discharge? During severe storms 
of rain, the water discharged over the land to a very large extent 
finds its way at once into brooks and rivers, by which it reaches the 
sea. Mr. David Stevenson remarks that, according to different 
observations, the amount carried off in floods varies from 1 to 100 
cubic feet per minute per acre.’ In estimating and comparing, 
therefore, the ratios between rainfall and river discharge in different. 
regions, regard must be had to the nature of the rainfall, whether it 
is crowded into a rainy season or diffused over the year. Thus though 
floods cannot be deemed exceptional phenomena, forming as they do 
1 Lucas, Horizontal Wells, London, 1874, pp. 40,41. See also Braithwaite, “ On the 
Rise and Fall of the Wandle,” Minutes Proc. Inst. C.H., xx. 
2 In the present state of our information it seems almost useless to state any of the 
results already obtained, so widely discrepant and irreconcilable are they. In some 
cases the evaporation is given as usually three times the rainfall ; and that evaporation 
always exceeded rainfall was for many years the belief among the French hydraulic 
engineers. (See Annales des Ponts et Chaussées, 1850, p.383.) Observations on a larger 
seale, and with greater precautions against the undue heating of the evaporator, have 
since shown, as might have been anticipated, that as a rule, save in exceptionally dry 
years, evaporation is lower than rainfall. As the average of ten years from 1860 to 
1869, Mr. Greaves found that at Lea Bridge the evaporation from a surface of water was 
20946, while the rainfall was 25°534 (Symons’s British Rainfall for 1869, p. 162). But 
we necd an accumulation of observations, taken in many different situations and ex- 
posures, in different rocks and soils, and at various heights above the sea. (For a notice 
of a method of trying the evaporation from soil, see British Rainfall, 1872, p. 206.) 
* “Reclamation and Protection of Agricultural Land,” Edin., 1874, p. 15, 
