RAINFALL AND FLOODS. 25 



away. It matters not to our purpose what will be the total 

 extra discharge of water hj the river which will follow the 

 downfall of rain. What we want to know is simply the 

 maximum rate at which that discharge will take place. To 

 assume (as some engineers appear to have done) that in 

 order to obtain the rate of discharge it is only necessary to 

 ascertain the number of cubic feet of water that have fallen 

 over the area, to apply to that number a certain deduction 

 for absorption, and to divide the number so reduced by the 

 number of hours during which the rain fell, involves to my 

 mind a double error. It assumes that the rate of discharge 

 will be uniform, and it assumes that the discharge will 

 occupy the same interval of time that the rain occupied in 

 falling — assumptions both at variance w^ith observed facts. 



I have not at my command such a series of measurements 

 of the height of water in the Frome after heavy rain as 

 would prove this point, and I am aware that the phenomena 

 of one river may differ very much from those of another 

 river. But I have been favoured by a correspondent with 

 some observations made on the height of the flood-water 

 in the Avon above the Frome, which are interesting in 

 themselves, and are sufficiently germane to the matter in 

 hand to warrant their introduction here. They refer to the 

 heavy rains of October last, and the observer is Mr. H. 

 C. Martin. 



From October 5th to October 22nd the fall of rain was 

 almost exactly 8 inches. Between Oqtober 23rd and Novem-. 

 ber 7th the entire fall was only a quarter of an inch. The 

 rain may therefore be said to have ceased on October 22nd. 

 Yet the fresh in the river Avon, which reached its highest 

 level on October 23rd, had not entirely subsided on Novem- 

 ber 7th, sixteen days from the cessation of the rain. It 

 may be that in the Frome the period of subsidence of the 



