968 THE HONGKONG GOVERNMENT GAZETTE, 27TH SEPTEMBER, 1890.
centrate the flow through them, so as to produce a brisk current, thus preventing deposit, either in the out-fall pipes or at their mouth. Fresh sewage is very rapidly assimilated by fish and crustacea, and as it will be delivered into from 10 to 30 feet of water, it will be so diluted as to produce no nuisance, certainly nothing comparable to that inevitably caused by the mass of craft which frequent the Praya. The proposed out-fall, therefore, is free from objection.
28. The sizes of the sewer-pipes, as proposed by Mr. COOPER, are certainly ample; and will suffice to convey, not only the sewage-proper but a considerable amount of rain-water also. Indeed I think it possible that it may prove desirable to reduce the diameter, in some instances at least. But on this point I cannot give an opinion until I have made the necessary calculations; a somewhat lengthy operation, which would have unduly delayed the submission of this
report, had I waited for their completion.
29. No hard-and-fast rule can be laid down as to the position of ventilating openings; each case should be decided on its merits. They should be as far from dwellings as practicable. The middle of the road should be avoided, aud above all, they should be as inconspicuous as possible.
30. Mr. COOPER estimates the probable cost of the works for this district at $19,000. I am of opinion that this sum will amply suffice to cover the cost of the necessary works. For reasons already given in a previous report, I think it desirable that the two nullahs which bound inland lots Nos. 941 and 946 should be regulated, in the manner described in the said report. The cost of the work may be estimated at $6,000 making a total of $25,000.
31. Unless the Surveyor General requires the larger part of the sum of $50,000 already voted on account of E. V. 2 main-drainage and sanitary, the works may be commenced under authority of this vote, as soon as the necessary specifications and working-drawings can be prepared. I recommend, therefore, that authority be given to commence this work as soon as possible. I think it most important that some part of the Drainage Work should be in hand, before I leave the Colony. If an opportunity be thus afforded me of seeing work in progress, I shall be much better able to advise and direct future operations, as Consulting Engineer, from home, than if I merely left a set of plans and specifica- tions with the Resident Engineer.
32. I propose shortly to report to you on the drainage of a second section of the city, between the High-level District treated in this report, and the sea, and bounded eastward by the City Hall. Lastly, I propose to report on the drainage of the flat portion, to the eastward of the City Hall.
To
The Honourable THE COLONIAL SECRETARY.
I have the honour to be,
Appendix I.
Sir,
Your obedient Servant,
OSBERT CHADWICK.
Note on the evolution of gas by sewage.
Experiments made by the late Dr. LETHEBY on the generation of sewer-gas from sewage, show that a gallon of sewage containing 128-8 grains of organic matter, when excluded from the air, gave, in "nine weeks 12 cubic inches of gas per hour, consisting of 73·833 of marsh gas 15:899 10-187 of nitrogen, and 0-081, of sulphureted hydrogen." It is to be noted that this is the result of a laboratory experiment, and after decomposition has purposely been allowed to take place.
(BALDWIN LATHAM, Sanitary Engineering, page 325.)
The sewage of Hongkong will probably amount to about 3,000,000 gallons a day. If this were collected in a tank and kept till it was in a putrid condition, it would evolve according to Dr. LETHEBY's experiment, 50,000 cubic feet of gas per day or sufficient to fill a room 37 x 37′ x 37.