RELATION OF DENSITY TO PERCENTAGE 71 



In some German practice it is customary to deduct such surfaces 

 from the contributing area. 



The various kinds of relatively impervious surface found 

 on urban territory were classified by Kuichling as follows: 



1. The different varieties of roofs from which nearly all 

 water runs off. 



2. The first-class sidewalks and pavements, such as asphalt, 

 and cut-stone blocks or brick with asphalted joints. 



3. The second-class sidewalks and pavements, such as the 

 common Medina blocks with large open joints. 



4. The third-class sidewalks and macadam or gravel 

 pavements. 



5. Ordinary graded roadways and similar surfaces. 



From the best pavements and sidewalks a considerably 

 less proportion of water is discharged than from roofs because 

 of the irregularities of surface and because of the absorption 

 by the dust and dirt, even if the surface itself is practically 

 non-absorbent. The other classes, of course, retain a still 

 larger percentage, owing to deeper depressions and ruts and to 

 the greater absorptive power of the material itself. 



By an analysis of the conditions in cities like Buffalo, Syracuse, 

 and Rochester it was found that in well-developed city dis- 

 tricts there are on an average 32 persons per acre.* With an 

 assumption of 5.6 persons per dwelling, there should be therefore 

 about six dwellings per acre in such territory. In the cities 

 investigated it was further found that about 27 per cent of the 

 entire area was occupied by public streets and alleys, of which 

 43 per cent, or one- tenth of the entire surface, was provided 

 with some kind of pavement varying in quality with the char- 

 acter of the district. In the growth of cities this proportion 

 is likely to increase, it was observed, until all of the 27 per 

 cent has some more or less impervious pavement. A certain 

 roof -area was assumed for the six dwellings, and that for an 

 assumed business block or tenement added, with something 

 more for possible barns or sheds for each acre, the result being 



* In Ithaca, N. Y., by actual count there are 26.2 in the residential district. 



