EAINFALL AND FLOODS. 23 



These considerations obviously bear chiefly upon questions 

 of water supply, but it is necessary to introduce them here 

 in order to point out the differences to be observed in the 

 discussion of questions of water supply on the one hand and 

 questions of flood on the other. 



In discussing a question of flood, evaporation may be left 

 out of account altogether, for the amount of evaporation 

 during rain is practically nil. Absorption demands con- 

 sideration, no doubt, but in quite a different sense from that 

 in which evaporation has to be considered in a question of 

 water supply. The loss of water by evaporation in a ques- 

 tion of water supply may be expressed with approximate 

 accuracy in figures, and, if necessary, may be reduced to a 

 percentage. The effect of absorption on a particular rainfall 

 in a question of flood admits of no such treatment. The 

 proportion of rain which is held back by the soil and 

 strata will depend entirely upon the previous condition of 

 these in regard of moisture ; it will therefore differ greatly 

 in different cases, and can only be roughly estimated by a 

 reference to the weather which has prevailed during pre- 

 ceding days or weeks or months, or, with less trouble, by 

 a reference to the state of the river before the rainfall, 

 which state will faithfully tell the tale of the previous 

 weather. 



It may be said that, to ensure safety from flood, we should 

 assume the extreme case of complete saturation of the soil 

 and strata at the commencement of the rainfall. But com- 

 plete saturation cannot be except after long-continued and 

 heavy rain, and therefore cannot exist at the commeyi cement 

 of a rainfall, unless by an arbitrary arrangement we exclude 

 from the rainfall which we take as the basis of our calcula- 

 tion such portion of it as we may consider sufficient to 

 saturate the ground. But any such proceeding would (as 



