SEWER PLANS 205 



author that both of these factors of safety are not necessary, 

 and that, except in rare cases where the pipes are expected to 

 run full, where the cutting is deep and the line long, this require- 

 ment of grade can be omitted. 



The Rawlinson principle of straight lines between manholes 

 is rigidly insisted upon except for sewers large enough for a 

 man to walk through, and all curves and changes of direction 

 are made in the manholes. It has been recently recommended 

 that even the house-drains, which are generally made to enter 

 the sewer through a Y branch, should connect through a T in 

 order to facilitate inspection. The direction of the flow imparted 

 by a Y branch is said to be imperceptible, especially when the 

 branch has a fall of 6 inches or more, and the possibility for 

 inspection is very desirable. 



To compensate for the increased resistance to the flow in 

 the short curves made in the manholes, it is usual to add a small 

 fall in this curve, amounting to an inch or so for an 8- to 1 5-inch 

 pipe. 



Recently in one of the engineering periodicals, there has 

 appeared a theoretical consideration of the actual loss of head 

 incurred by the flow of sewage around such curves. The varia- 

 tion in the conditions such as the relative height of the flow 

 line in the main sewer and in the lateral, and in the clean con- 

 dition of the pipes is such that no exact solution can be hoped 

 for. Probably an inch fall in the bend is more than enough, 

 but it does no harm and prevents deposition of sediment at the 

 end of the lateral. 



In order that the streams from two or three sewers meeting 

 at the same manhole may have as little eddy-forming effect as 

 possible and may meet and continue to flow with the least 

 deposit of sediment, it is desirable that the streams all have the 

 same velocity in order that in no one shall the velocity be 

 checked. It is desirable also that the sewage-level in each of 

 the joining sewers shall be at the same height. This last of 

 course is not possible for all stages of all intersecting lines, but 

 it may be so for the depth of flow for which the sewers are 



