208 



SEWER DESIGN 



depths is considerable, although the velocity can never be made 

 constant, since with the same grade the larger circle will have 

 the greater velocity. 



Many different shapes have been tried, though there are now 

 but two in common use for this purpose. The egg-shape 

 shown in Fig. 57 was introduced in England by Mr. John Phillips 

 in 1846, and is used to-day with the same proportions then 

 advised. The vertical height is equal to one and a half times, 



FIG. 57. 



the radius of invert is equal to one fourth, and the radius of the 

 sides to one and a half times the transverse diameter. The 

 other form of egg-shape, Fig. 58, has a smaller invert and is 

 therefore better adapted to sewers where the depth of flow may 

 at times be very small. The vertical height is one and a half 

 times the transverse diameter as before. The radius of invert 

 is one-eighth of the transverse diameter, and the radius of the 

 sides one and a third times. Latham says that this new form 

 is stronger than the old, and that with small volumes of flow 

 it is better adapted to be self -cleansing than the earlier form. 



