44 
laid through the streets. These ejectors are most unique in their 
mechanism. 
I consider this system the best of all the pneumatic systems, 
as it may be applied to the whole or only a part of the drainage 
of a town, however large or small; to the wHone where the 
topography is such as precludes drainage by gravity, or to the 
PART where partial gravity can be obtained. 
Water CARRIAGE. 
On this head not much need be said, as all are familiar with 
the system. All know the action of the ordinary water closet, 
flushed at will by the person using it. Connected directly with 
the main sewers, the contents are at once removed by gravity and 
immediately conveyed to the discharge point, and there dealt 
with. Generally the flow of the sewage is quick, as although 
some sewers may be flat and sluggish, still the majority will be 
laid on good gradients which will ensure a quick flow throughout 
the whole of the sewers, thus reducing the amount of gas to a 
minimum. ‘To guard against the gases arising from the sewers, 
is a question which occupies the mind of the engineer, and often 
puts him in great difficulties. It is one of the hardest things to 
thoroughly ventilate sewers, and to steer clear of all public censure. 
Now the ventilation of sewers is necessary for two great reasons. 
1st, from a general sanitary point of view, and 2nd, to prevent 
the gases in the sewers being suddenly compressed. This may 
be caused by any increase in the volume of sewage (as may occur 
during sudden storms), or by the effect of any rapid rise of 
temperature in the sewer, as may be caused by the admission of 
hot water or steam. A very slight degree of pressure is enough 
to overcome the resistance offered by water-seal traps, which are 
the usual and best defence against the entrance of sewer gases 
into our dwellings. For this reason it is wise to be very guarded 
in allowing hot water or steam to be turned into the sewers of any 
town. Many plans have been suggested for ventilating sewers, 
such as the use of furnaces and high chimneys, found quite 
impracticable, and thus reported on by Mr. W. Haywood, engin- 
eer to the London City Commissioners of sewers :—‘‘My im- 
pression is, that there will be exceeding difficulty in doing it at 
all; but if done, it will be at an enormous yearly expense. The 
difficulties to be overcome are gigantic. . . . As there are 
thousands of openings to a sewer, all these openings are continu- 
ally varying in their condition, and you must bear in mind that 
supposing the whole district had been so arranged as to give the 
furnace and flue a sufficient exhaustive power, the mere opening 
of a water closet, or the enlarging or putting in of a new drain 
into a sewer, or the making of a hole a foot square, would very 
much destroy the power of the furnaces, and unless you had a 
