RELATION OF DENSITY TO PERCENTAGE 73 



roof-surface and improved street-surface and paved yards 

 seldom reaches 100 per cent, as there are always a few open 

 spaces, gardens, small parks, etc., so that for areas of any 

 magnitude, even in the largest cities, the limit may be set at 

 90 per cent. The street-area cannot exceed 27 per cent of the 

 entire area unless yards be included, when it may amount to 

 40 per cent; and the roof-area will reach 60 per cent, as a 

 maximum, for cities like Rochester. 



But the paved streets are not absolutely impervious, only 

 relatively so, and the hard-earth yards, while allowing some 

 rain to run off, also retain some, so that the percentages given 

 above are only of the areas to be considered. It remains to 

 determine what proportion runs off from the four classes. The 

 loss of water by absorption and evaporation from roofs is gen- 

 erally so small in heavy rains that it may be neglected, so 

 that the roof -surf ace may be taken as truly impervious. As 

 to the percentages furnished from pavements and sidewalks 

 the amount varies with the quality of the pavement; and while 

 no record of exact experiments was available, it was estimated 

 that from a well-paved stone or asphalt pavement 80 per cent 

 of the rain ran off. From well-kept macadam or gravel roads 

 from 30 to 50 per cent of the rain was obtained, and, inter- 

 polating for other pavements, for second-class sidewalks and 

 stone pavements the discharge would be 60 per cent; for the 

 best macadam, 50 per cent; and for inferior macadam and gravel 

 roads not more than 40 per cent would reach the sewers during 

 a hard storm. The proportion to be expected from the hard- 

 earth surfaces of streets and yards is evidently subject to 

 great variation, but it was assumed that it would be 20 per cent 

 of the rain falling. 



Correcting Table VIII by these percentages of discharge, 

 and assuming, as indicated by the Rochester studies, that the 

 quantities of the different classes of pavement were divided 

 as given, in proportion to the different densities, we obtain the 

 following table: 



