ANNIVERSARY ADDRESS. 43 
the process, :thus providing for dilution and oxidation of the noxi- 
ous gases, the work being completed by delivering this mixture 
above the tops of the houses. 
This system is acombination of the plenum and vaccuum systems, 
the fresh air is forced into the sewers by means of a tall shaft 
fitted with an induct cowl, and considerable advantage is gained 
by this method, as the air is purer than if drained from a lower 
level, the oxydising effect of air so obtained being apparent, the 
sewer air becomes diluted, and the temperaature of the sewers so 
ventilated approximates to that of the external atmosphere. The 
diluted air is exhausted by tall shafts carried up the walls, and 
well over the roofs of the houses so that air is distributed far 
beyond the limits of danger. From observations made at the — 
different shafts during the year over the lines of sewers, operated 
upon by the Board, which aggregates seventy-four miles of sewers 
of varying sizes, it was found that the average work done by the 
induct shafts (or air forced into sewers by wind power) varied 
from 1,397 to 4,554 cubic feet per hour. The results are derived 
from seven hundred and forty observations, the average wind 
velocity being 8°43 miles per hour. The total amount of work 
done, in withdrawing air from the sewers by the whole of the 
exhaust shafts erected, is equal to 1,579,656 cubic feet, or 56°41 
tons of air, and the total amount of pure air passed into the 
sewers by the induct shafts is equal to 857,400 cubic feet, or 
30°62 tons of air; the oxydising effect of this supply of pure air is 
apparent when it is considered that twenty-one parts in every 
hundred consists of oxygen. In addition to the induct shafts, 
the Board have also under test a system of forcing air into the 
larger sewers by water sprays, so far with satisfactory results. 
The quantity of air introduced per cubic foot of water is equal to 
1842:55 cubic feet. A proper adjustment of the grades and sizes 
of sewers with a good system of ventilation are necessary in order 
to secure the objects aimed at—even flow of the sewage, and 
motion and oxidation for the sewer air. 
