VENTILATION OF SEWERS AND DRAINS. 149 
evident that the air which occupied the upper half of sewer will 
be compressed into half the space it originally occupied. Accord- 
ing to the law of Boyle and Mariotte, the pressure is inversely as 
the space occupied ; considerable force will therefore be brought 
upon the traps in the vicinity of the sewer, which would unless 
openings are provided, be forced; this danger is however reduced by 
the porosity of the materials with which the sewers are constructed; 
the openings in the material are so minute that currents of air 
will pass through which may escape attention. A subsidence 
of the water to its normal flow will cause a reverse action. If the 
sewers are ventilated direct to the crown of the road the sewer 
air would be forced out into the street and become so diluted as 
not to be dangerous although unpleasant. 
The fluctuation of the flow of daily sewage has the effect of 
compressing or dilating the air, as well as leaving the sides 
between maximum and minimum flow lines wet with sewage, this 
leads to the formation of vapour and gases. This fluctuation has 
been used as an agent in ventilating sewers into the roadway, it 
was contended that the alternate expelling and drawing in of air 
would set up sutiicient current to provide for diluting and oxydizing 
the air in sewer. In other words, sufficient openings were to be 
made to allow the sewers to breathe. 
The difference between the temperature of the sewers and the 
atmosphere at certain periods of the year is a powerful agent of 
natural ventilation. The air being at certain seasons expelled 
from the highest shafts and cold air drawn in at the lower ones, 
in other seasons the reverse action takes place. This upcast and 
and downcast action is due to the difference in weight of the air 
columns ; there are some months in the year when the external 
and internal temperatures approach equality, at such times there 
is no movement in the air—except currents are set up by the air 
in the sewer being compressed by influx of water. 
It has been found that the earth temperature controls the 
movement of air to some extent. Observations extending over a 
period of eighteen months have been made by an officer of the 
