284 THE RAINFALL IN 1896 DOANE. 



two-thirds full. The designer made a liberal estimate in deter- 

 mining the capacity required, and yet during the past year it 

 was plainly demonstrated that the capacity of the sewers was 

 not sufficient to carry off the rainfall, and great trouble, damage 

 and inconvenience has been caused in consequence. There is not 

 the slightest doubt that the greater part, if not the whole, of 

 this trouble would have been obviated if records of self-recording 

 rain gauges had been available. While the greatest rainfall on 

 record in 1876 was .183 inches in half an hour, we had in 1896 

 a storm lasting 7.5 hours, with an average rate of fall of .52 

 inches per hour, and another lasting 3.8 hours, with an average 

 fall of .92 inches per hour. The maximum rate must have been 

 greatly in excess of even the latter figure, but as the storm came 

 n the dark hours of the morning, and the rain was not measured 

 by self-registering instruments, we can only guess at the maximum 

 rate per hour. 



The design of sewers depends principally on two classes of 

 storms. These are short storms of great rates of precipitation, 

 and long storms of ordinary rates of precipitation. It is not 

 sufficient to know the rainfall per hour. The severity of a 

 storm often reaches a maximum during from 10 to 20 minutes 

 only, and this maximum should be determined, if possible. It 

 is also most important that the local conditions of the surface 

 should be known. If the ground is saturated before the storm 

 the rainfall will run off more rapidly. 



A chief purpose to be subserved by a rainfall record is not 

 merely how often does the maximum rainfall occur at each point, 

 for that is an event which only occurs once or twice in a century. 

 The great desideratum is : How often do the heaviest rainfalls 

 of various rates occur, and for how long a maximum and average 

 time does such a rainfall continue ? The records from which 

 such laws are deduced must necessarily be somewhat voluminous, 

 and yet by proper study, aided by records of a number of years, 

 a very close approximation to the real probabilities could be 

 obtained and drawn graphically on charts, which would be of the 

 greatest aid to hydraulic and city engineers ; and even without 



