XL. J. I. HAYCROFT. 



The kind and condition of soil is a most important factor 

 in dealing with this subject. For example, during a period of wet 

 weather of, say, four days' duration, and of varying intensity, 

 the run off from a given area will vary considerably in the 

 extreme cases of a porous area, such as a sandy surface, and an 

 area consisting of a clay surface. In the first case the four days' 

 rain may never sufficiently saturate the soil to permit any of the 

 rain to now off during that period in such quantity as to need 

 any provision but the smallest culvert, although the flow may 

 continue for some weeks after the rain has ceased, whilst in the 

 case of a clay surface, even after a period of drought, the area 

 may be rendered impermeable, after, say, one day's rain, so that 

 all the fall for the following three days must be provided for, 

 while a day or two after rain ceases the creek in which the run 

 off took place would practically be dry again. 



In this connection it may be said that maximum rainfalls 

 should be considered as falling on impervious ground, inasmuch 

 as such rainfalls frequently occur during a continuance of wet 

 weather. The experience of the writer is that such heavy rain 

 falls form climaxes to periods of wet weather, which, commencing 

 moderately, proceed in intensity (not, it is true, in any fixed 

 ratio), come to a head in a very severe rainfall of short duration, 

 and then fall in intensity gradually, until rain ceases for that 

 particular period. 



Now, the difficulty with which an engineer has to contend is 

 that rain gauges, except those automatically registering, show 

 nothing but the gross amount of rainfall between the periods of 

 observation. As, for instance, a rain gauge observed at stated 

 intervals, say 24 hours, showing, perhaps, that during that period 8 

 inches of rain had fallen, does not indicate whether 7 inches of it 

 fell in 23 hours and one inch during the balance, or vice versa. In 

 such a case experience is of value in forming a correct judgment, 

 and the most severe conclusion which may be drawn from 

 observing such a gauge, guided, of course, by what has been 

 learnt in regard to such rainfalls, either from recording gauges or 

 from experience, will be the safest. The records of a self- 



