(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.
(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.