(_b._) The position of the head of the rain-water pipe is generally the worst that could be chosen for the egress of the gases, both on sanitary and pneumatic grounds. (_c._) The joints of a rain-water pipe are usually none of the best. (_d._) The objections persons naturally have to allow the rain-water pipes of their houses to be used for such a purpose. (4.) By utilising the lamp posts or columns adjacent to the sewer. The objects of this system are first to obtain a constant upward current from the sewer, and secondly to secure that the foul gases and air shall be consumed and rendered innocuous by being burnt. The objections to this system are-- (_a._) The lamps are only lit at night, consequently little or no ventilation would be going on during the day. (_b._) The number and diameter of the lamps are generally too small to make any appreciable effect upon the ventilation of the sewer. (5.) By passing or filtering the foul air through charcoal placed in trays or other receptacles in shafts. The theory of this method is admirable, as the charcoal would arrest all the impurities and flocculent organic matters contained in the foul air, and allow only the pure filtered air to pass into the atmosphere. It has been found, however, where this method has been adopted, that the charcoal very soon becomes so caked and consolidated from damp and the vibration of the traffic, that it will neither allow the air to filter through, nor absorb the impurities contained in it; these objections and the amount of attention this system requires, has not rendered it very popular with town surveyors, although it has much to recommend it. (6.) By making use of ordinary chimney shafts. This method and the one which follows have some merits, the principle being that an upward current is established as an exhaust from the sewer, and also that the foul air is purified by being passed through fire, but both these advantages are only gained when the fires are actually burning; the objections to this system are-- (_a._) Structural difficulties must often be encountered and overcome. (_b._) Possibility of explosion arising from leaks of gas mains into sewers (a by no means uncommon occurrence, as all who have charge of sewers frequently find to their cost). (_c._) The objections of owners and occupiers to allow their premises to be thus made use of to carry off a public nuisance for which they as individuals are in no way responsible. (7.) By lofty shafts erected at convenient positions which are either in connection with furnaces or are simply open to the air.