XLII. J. I. HAYCROFT. 



as they occur in Nature — viz., the whole area affected by a 

 rainfall of moderate intensity, during continuance of which 

 occurs a maximum rainfall on portion of the area. Such cases 

 occur, and are those which cause maximum volume at an outlet, 

 and, therefore, are those to be provided for. 



The area of the catchment. — This item, of great import- 

 ance, can always be determined with a sufficient degree of 

 approximation, either by marking the line of watershed on a 

 reliable map, or, in the absence of such, making a rough traverse 

 thereof. It has been already stated that all formulae are limited 

 to definite areas, but the determination of the extent of the 

 maximum area to which a formula may be considered applicable 

 is a very difficult matter. It has already been said that when the 

 run-off from a catchment area takes place in a defined channel, 

 formulae are unnecessary. No definite rule applies as to when 

 formulae are to be put on one side, and flood-marks made use of. 



In connection with the subject of flood-marks there are certain 

 liabilities to error, to which attention may be drawn. Jackson's 

 remarks have already been referred to, but the writer calls to 

 mind a case in this colony where a catchment of nearly 50 square 

 miles, with a well-defined creek as an outlet, was observed by 

 him within six days of rain having fallen as absolutely dry, and 

 this in a country where one day's steady rain would render the 

 surface practically impermeable. Flood-marks were observable 

 on the railway bank, through which the culvert forming the 

 outlet for this area was constructed, but such marks were useless 

 to determine the discharge from that area, even if the rainfall 

 were known, as, if the line had not existed, their height would have 

 probably not been within 18 feet of where they appeared to be. 

 Observation of flood-marks, to be of practical use, should be 

 taken at such points on a stream where the flood-waters are 

 removed from any chance of being unduly raised, owing to 

 obstructions, insufficiency of structure, or other causes having a 

 like effect. 



The longitudinal section in the case mentioned showed the 

 inclination of the bed of the stream above the obstruction as 



