PROPORTION REACHING THE SEWERS 57 



was required in each case before a change in the rate of rain 

 was manifested in the outlet. In preparing the design for the 

 East Side Sewer an extensive series of observations were car- 

 ried out, containing valuable data and contributing largely to 

 our knowledge of the subject. The four rain-gages, already 

 alluded to, gave him as accurate a knowledge of the rainfall 

 as was possible without automatic gages. Simple self-recording 

 gages were placed in the principal outlet sewers of the East 

 Side, and the cross-sections, dimensions, and slopes of those 

 sewers were all carefully determined. It was noted even with- 

 out the gage-reading that slight variations in the rate of pre- 

 cipitation were quickly felt in the sewers, and the flood-heights 

 therefore were due to the maximum intensity of the rain, 

 usually lasting but for a few moments, and not to the average 

 intensity for the whole period of the storm. Moreover, the 

 periods of maximum intensity of rainfall corresponded closely 

 with the period of maximum discharge, and in a rain of varying 

 intensity the volume of sewer-discharge followed the rain in 

 parallel waves. 



The drainage-areas were carefully determined, so that the 

 actual volume of the ram falling was obtained, and the amount 

 discharged was calculated by Kutter's formula from the height 

 of flow in the sewer, as shown by the gage, and from the hydrau- 

 lic slope of the sewage. During 1888, 17 storms were gaged, 

 their intensities varying from 0.24 inch to 3.20 inches per 

 hour in the different sewers. A summary of the results is given 

 in the following table, for which Kuichling claims no great 

 accuracy, since the amounts of the intermediate showers were 

 not always well known, though the totals are reliable. They 

 are well worth regarding, however, as being the only careful 

 records of the relation of rainfall to sewer-discharge that are 

 available. 



It can be seen on inspection that the discharge from District 

 X is invariably the largest, accounted for by the fact that it 

 has the largest proportion of roof-surface and other impervious 

 ground-covering. The effect of a light rain immediately pre- 



