LXII. DISCUSSION. 



of this table, however, proved very interesting. Thus, 

 taking Johnstone's Creek area, the rainfall provided for was 



1 ^ inches per hour, and the Rushcutter Bay area, the rainfall was 



2 inches per hour. He was aware that in the latter case 

 the increase in rainfall was made on account of the relative 

 smallness of this area compared with the former ; but Mr. Davis, 

 in his practice, seemed to take no notice of the nature of the 

 catchment. In the former case the catchment would be rendered 

 practically impermeable after a gentle, soaking rainfall ; whilst 

 in the case of Rushcutter Bay, the nature of the catchment was 

 such that, after several days' rain, very little would find its way 

 into the channel. Mr. Davis stated that these channels had 

 been constructed in every part of the city (Sydney) ; but 

 he had yet to learn that any of those given in the table were 

 within the city boundaries. Mr. Ross's remarks were apropos, 

 and valuable, as coming from an observer, who, like the speaker, 

 had to deal with such questions in his daily practice. Mr. 

 Cardew's opinion of the table of discharges in the paper was 

 fallacious, as the very information which he said was not given 

 was stated very fully on the diagram, accompanying the paper, 

 which was compiled from the table referred to. Mr. Cardew's 

 remarks on the subject of the duration of rainfall were entirely 

 novel, and personal to himself. According to Mr. Cardew's 

 statement, if the duration of a rainfall were not sufficiently long 

 to fulfil his conditions, maximum flow would not occur at the 

 outlet. Nothing could be more absurd. The direction, and not 

 the duration, of the storm was the controlling factor. A storm 

 travelling in the same direction as the water flows off a catch- 

 ment towards the outlet, would cause a flood of much greater 

 volume (and in particular cases the maximum volume for such a 

 catchment) than the same rain storm travelling in an opposite 

 direction, or one at right angles to the course of flow-off, though 

 in each case there was the same amount of rainfall. Again, the 

 duration of rainfall by Mr. Cardew's method was pure guess 

 work, which should be eliminated from all rational formulae. In 

 conclusion, he desired to thank the various members for their 



