The ventilating pipes and shafts must not be less in any case than the sectional area of the drain with which they are connected. In some instance the air-inlet upon the house side of the trap is an ordinary pipe-shaft protected l>v a cover. A chamber or manhole, while rather more costly, is, however, of much greater value If a shaft is used, special attention must he called to the necessity of its being a clear-way shaft, free from bends or angles, as these intercept large percentage of the volume of air passing through, and the grating or cover which is fitted upon the shaft must have openings between the bars equal in the aggregate to the sectional area of the pipe or shaft. It is also provided in the by-laws that such gratings shall be fitted with appliances for preventing the passage of dirt or other substances into the drains, and suitable covers have been designed for the purpose. One of these special covers, which has been largely used, is the well-known "Cregeen " cover, shown in Fig. 332. It readily and securely fitted to the socket of the air-pipe; the dirt-l>ox is formed around the cylinder, none of the air-openings in the cover being above the mouth of the pipe. This cover is made in two ways; the one being of cast-iron of strong section for use in situations traversed by heavy traffic, and the other haying the dirt-box formed of stoneware, the iron-cover being loose; this is quite suitable for situations free from traffic Another very suitable cover, manufactured by Ham Baker & Co., has a loose dirt box in the form of a tray. The section of this cover is shown in Fig. 333.

Fig. 332   Cregeen's Air Inlet Cover.

Fig. 332 - Cregeen's Air Inlet Cover.

Fig. 333.   Ham Baker & Co.'s Air inlet Cover.

Fig. 333. - Ham Baker & Co.'s Air-inlet Cover.

In Knight's annotated edition of the Model By-laws there are illustrations showing two methods of air-inlet, the one in the street- foot path and the other in the curb of this footpath. There are very strong objections to these methods. In the first place, the scruples have to be met of those people who have a horror of seeing any opening connected with a drain; and secondly, dirt boxes in such a position are liable to be tampered with, and sanitary authorities are very chary of permitting them to be so fixed. In special circumstances like these, it is preferable to construct the air-inlet as shown in tig. 334, the air-inlet pips being fixed on the face of an adjacent wall, or if necessary, inserted in a chase formed in it. The air-inlet itself should be out of reach of any passers-by. (say) at least eight feet high, and may be fitted with a mica flap valve to prevent any outrush of air.

A disconnecting chamber or manhole within the curtilage of the premises and as near to the public sewer as possible. is infinitely better than a simple air-inlet pipe. The size will depend largely upon the number of branch-drains connected with it, but there should be plenty of room for access to the disconnecting-trap for inspecting the drain, and for cleansing it whenever required. The entrance-cover may be provided with an open grating to serve as an air-inlet, or it may be found preferable to use an air-tight cover, constructing an air-inlet in the manner shown in the last figure. Fig. 335 shows a section of a typical disconnecting chamber. In the drainage of town buildings, especially office premises, it is difficult to provide within the curtilage a disconnecting chamber, so that this will have to be constructed under the footway, subject to the approval of the local sanitary authority, who will no doubt not only exact a payment for the easement.

but will also impose conditions as to the size. the means of access, and the projection of the air-inlet duct These, of course, will in a large measure be determined by the importance of the thoroughfare, the width of the footway and the amount of traffic passing over it; therefore, no bard-and-fasl line can be laid down, but it may serve as a guide to illustrate and explain some constructed under my directions a few years ago in Manchester round one of the largest banks, no greater projection being allowed than 1 foot 8 inches, and the length being fixed at 3 feet 6 inches. This sixe will just allow a man when kneeling to get access to the trap. A bonnet-arch (see Fig. 386) was turned 5 feet above the invert, and a shaft 20 inches square carried to the surface. The cover was an air-tight iron cover, but, instead of having a studded plate, ribs were cast across, and it was filled with concrete rendered smooth, so as to be similar to the pavement. No projection of the air-pipe was allowed, so a chase was rut into the wall, and a pipe 8 inches by 6 inches. inserted and secured to the wall by ears sere wed into plugs. The grating was 8 feet above the footway. A number of waste-pipes of all kinds discharge into these chambers, and although to comply with the Corporation's conditions was necessarily an expensive piece of work, the chambers were satisfactory to all parties, and they have continued to work well.

Fig. 334.   Air Inlet to Drain fixed to Wall.

Fig. 334. - Air Inlet to Drain fixed to Wall.

Fig. 335   Section of Intercepting Chamber with Air Inlet Cover.

Fig. 335 - Section of Intercepting Chamber with Air Inlet Cover.

Fig. 336   Manhole for Towns where only small space is allowed

Fig. 336 - Manhole for Towns where only small space is allowed.