772 PROCEEDINGS OF SECTION H. 



diameter, with a fall of 1 in 3100). To seoiire this gradient 

 the sev^^er will probably have to be laid at considerable depths 

 under the surface. (Mr. Manserg'h ])roposed to construct 

 one part of his sewer at depths varyin<>' from 62 to 28 feet, 

 and tlie other part at a maximum depth of 8b' feet below 

 the surface). Now% if to carry out some other system of 

 sewage disposal a gradient of 1 in 100 could be obtained, 

 the same quantity of sewage could be carried by a 5-feet 

 barrel sewer ; and if this other system did not divert the 

 sewage from its natural outfall, it is probable that the better 

 gradient could be obtained with a sew^er laid at a compara- 

 tively small depth. If this were so, two benefits would 

 result : in the first place, the economic benefit of the saving 

 in the cost of the construction of a small sewer at a shallow 

 depth instead of a large sewer at a great depth ; and in the 

 second place, the hygienic benefit of having a more easily 

 flushed and more easily ventilated sewer, and one which is 

 not so capable of generating and storing sewer gas. For in 

 hot dry weather the larger sewer when compared with the 

 smaller exposes about double the surface of sewage to the 

 air for four times longer a period wherein to develop putre- 

 faction ; has about four times the storage capacity for the 

 foul air generated ; is at least four times more difficult to 

 ventilate; and requires at least four times the quantity of 

 water to flush it — a cpiantity which in sewers of large size is 

 really prohibitive. All these observations in reference to the 

 main outfall sewer apply with almost equal force to the main 

 collecting sewers, one of which, for instance, in Mr. 

 Mansergh's scheme is 14 feet in diameter, and laid at a 

 great depth, and as it is larger than the outfall sewer I 

 presume it is laid at a still flatter gradient. 



On the other hand, a sewage farm for treating the sewage 

 of a city before its discharge into the sea has the advantage 

 of sufficiently purifying it and disposing of it at one opera- 

 tion ; for the irrigating process will have left the solid part 

 of the sewage on or in the land, whereas a chemical process 

 leaves this solid part in the form of sludge in a tank, whence 

 it has to be lifted and got rid of by a second operation. I 

 may say at once that I do not mean to claim for a sewage 

 farm the additional advantage that it not only disposes of 

 the sewage but disposes of it by utilizing it ; for, as far as 

 our experience in England and the Colonies is concerned, 

 this utilization is only productive of increased cost in the 

 disposal : that is, every pound gained in a year by a sewage 



