RELATION OF DENSITY TO PERCENTAGE 77 



vious surface diminishes until that surface reaches a limit of 

 80 or 90 per cent, corresponding to a density of about 75 per- 

 sons per acre. Beyond this density there can be no material 

 increase of such surface, since then the whole available area 

 becomes covered with pavements and buildings, and any 

 additional population is accommodated by crowding more 

 persons into the houses. It is also proper to remark that the 

 figures given refer only to certain average urban conditions 

 and are therefore subject to such modifications as may be 

 appropriate under different conditions. For example, in a 

 rapidly growing suburban village the amount of water delivered 

 from the surface twenty years hence may be very different from 

 what the present indications would show. The measured 

 amounts of water, in the case of Rochester, served to check 

 the assumptions made, and have shown that they are very 

 near the truth, so that there can be no doubt that the method 

 as given will furnish, except under very exceptional conditions 

 of building or surface, results nearer the truth than can be 

 obtained in any other way. 



PROBLEMS 



26. Assuming 5 persons per house, determine from a count of the 

 number of houses in a given district the number of persons per acre. Pace 

 the distances needed to determine the area. 



27. By noting where additional houses might be built and where 

 business blocks might be put, estimate the possible future density of popu- 

 lation of this same district. 



28. On a given area of 240 acres, 50 acres are under roof, 36 acres 

 are brick pavement, 15 acres are stone block, 12 acres are bituminous mac- 

 adam, and 60 acres are hard earth streets and yards. What is the per- 

 centage of reduced fully impervious surface in percentage and what 

 population per acre does it correspond to? 



29. In a part of Ithaca, population 26.2 per acre, the lots are 66' 

 X 132', and the blocks are 10 lots long and two deep (660' X 264'). Esti- 

 mate the size of a house and the amount of street surface in a block to 

 compare with Table VII. 



