PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 



580 



ncum, Jerusalem, Nineveli, Cartliag«\ and many other cities of Europe, 

 Asia, Africa and America, very fre(iueiitly adopted a combined system of 

 large trunk sewers, with tubular house connections, and availed themselves 

 of excessive water supply, for flusliing the main lines. Some of these 

 arterial sewers (as the cloaca ma.rima, at Rome, fourteen feet wide and 

 tliirty-two feet high), were of enormous size, and the manner in which they 

 liave gradually tilled up, while the tubes are still available, furnishes a 

 striking comment on the general practice of modern cities, which, as in 

 London, Paris, and New York, have, until recently, ignored the use of the 

 tubular system and abound in examples of inherent defects. The positive 

 and negative testimony on this point, establishing and confirming theor}' 

 by long continued experiment, has led to the more recent adoption of the 

 combined trunk and tube method of drainage, which, in time, will super- 

 cede all others, and which will be presented in detail after a notice of topo- 

 graphical arrangement. 



From what has been said of the character of sewage, which requires 

 disposal through sewers, it is evident that provision for rain-fall far exceeds 

 ordinary house sewage and street wash. Now, as rain-fall must be dis- 

 posed of through the natural depressions of the locality drained, and the 

 quantity is determined by the natural drainage area, the sewer system of a 

 city must therefore be adapted in its proportions to its entire drainage 

 area, and is in this way independent of buildings or population. We refer 

 here to the arterial arrangement, which is to be extended from time to 

 time until the whole district is provided for and populated, and must be 

 adapted to the greatest ultimate discharge. The lines and grades of these 

 arteries are defined by the natural ravines for the flow of which they be- 

 come substitutes. 



It follows, from this law of arterial flow, that sewage discharge naturally 

 finds its outlet in the river, or other water current which receives the rain- 

 fall, unless, for agricultural or sanitary purposes, it is artificially inter- 

 cepted; and the propriety and method of such interception becomes the first 

 study in the details of arrangement. 



The arterial sewers for the primary, secondary, and tertiary ravines of 

 the general district must be adapted in size and grade to the areas thej'' 

 severally and unitedly drain, and to a maximum storm-fall deliver}'. The 

 following table of sizes, grades and areas, calculated for a continuous 

 storm-fall of one inch per hour, is applicable to trunk lines, and accepted 

 among hydraulic engineers as a reliable bassis of arrangement, inclusive of 

 house sewage and street wash. 



Area in acres drained under a storm/all of one inch per hour. 



