(_a._) The foul air escaping into the public streets is often very injurious to persons passing a ventilator, and sewers are buried out of sight, but they are not out of mind so long as we are constantly and unpleasantly reminded of their existence. (_b._) It is found that a change of temperature either of the atmosphere, or of the air in a sewer, will seriously affect the action of a shaft, causing it sometimes to have upcast currents of air, sometimes downcast; the effect of this latter action, especially when it arises from the direction of the wind blowing over or into the shaft, is frequently to drive the impure gases contained in the sewer into the house drains, and from thence into the houses, unless they are so trapped and ventilated as to prevent it. (_c._) They are also affected by the fluctuations of the flow of sewage in the sewer, or by barometric changes in the atmosphere. (_d._) The situation of the open grids in the street is sometimes somewhat awkward for traffic, and horses will frequently shy at them, they also admit solid road detritus into the sewer unless they are protected by a catch plate of some description. (_e._) They are tempting places for children to play over, with what results may be imagined. (2.) Open shafts are carried up the sides (gable ends if possible) of buildings in the neighbourhood of the sewer; these shafts may be either open at the top, or be furnished with exhaust cowls. This system is sometimes employed in conjunction with inlet shafts at the sides of the street, in the manner shown by the drawing which follows: [Illustration] This method has the advantage over the first system I have mentioned of carrying the smells further from our reach, but it also has the following disadvantages: (_a._) The distance and the number of bends and elbows the gases have to traverse before reaching the external air. (_b._) The difficulty of fixing them just at the points where they are most required with reference to the gradients of the sewer, especially if they are to be constructed of such an internal diameter as will ensure their efficiency. (_c._) The great objection raised by occupiers and owners of premises against having them fixed on their premises, both on sanitary and legal grounds. (_d._) Their great expense. (_e._) The effect of weather upon their currents. (3.) By making use of the rain-water pipes from adjoining buildings. This method commends itself as being very economical, and the opportunities thus given for ventilation are so numerous. If sewers are to be ventilated at all[194] it would seem at first sight that there could not be too many openings from and into them, but this system has the following serious objections: (_a._) When raining, little or no ventilation can take place, and this is the very time, owing to the rising of the water in the sewer, that the gases should be allowed free egress if such is considered the best manner of dealing with them.