ENGINEERING CONSTRUCTION AND RAINFALL. XLI. 



registering rain gauge, maintained for a number of years in a 

 given catchment, are invaluable. Few catchments are thus 

 equipped, In the absence of such a gauge the records of the 

 nearest rain gauge, considered in relation to its proximity to the 

 catchment and the physical geography of the locality, is the best 

 substitute. In designing a means of egress for water from a 

 catchment the maximum rainfall of the district is, of course, the 

 chief consideration, but this must be considered in an intelligent 

 manner. 



Let a typical case be considered. Say there are two towns, X 

 and Y, about 40 miles apart, lying between which is a range of 

 fairly high hills, practically parallel to the bee-line between the 

 towns. Now suppose, during a period of wet weather, a rain 

 gauge at X registers at the rate of 2 inches per hour, and a 

 gauge at Y at the rate of 6 inches per hour, an engineer, in 

 providing outlets for catchments, embracing the range and 

 sloping ground at its base, is bound to consider those various 

 catchments as liable to be subject to the greater rainfall per 

 hour. The rainfall should not be graded for the several catch- 

 ments between X and Y, so that at a point midway between 

 these towns a rainfall of only 4 inches per hour would be con- 

 sidered. Such a treatment would be very convenient if it was in 

 accordance with natural law • but, unfortunately, such is not the 

 case. On the other hand, the rainfall on the range would 

 probably exceed the record of the gauges observed at its ends. 



Nor, having fixed the probable maximum rainfall, as in this 

 case at the rate of 6 inches per hour, is it permissible, 

 because rainfall may take 6 hours to travel from the extreme 

 point of a certain catchmennt to the outlet, to consider it as a 

 rainfall at the rate of 1 inch per hour falling for 6 hours. 



lb is very important to consider only those catchments whose 

 area permits them to be affected as regards maximum volume 

 produced at their outlets by such rainfalls. An area whose 

 extent renders it unlikely to be wholly affected with the 

 maximum rainfall of the district, can still be considered as suit- 

 able for the application of some formula, by taking the conditions 



