aT) were 
148 JOHN M. SMAIL. 
inside of a sewer, as it can be above ground. So long as the air 
in our sewers is allowed free communication with the outer air at 
frequent intervals of space, no difference can exist between the 
tension of the air within and without, but the moment we obstruct 
such communication, various causes begin to act either separately 
or combined as the case may be. 
Heat is one of the most powerful forces at work within our 
sewers, and is capable of developing a force which would be a 
source of danger in any unventilated system of sewer or house 
drains. During the day there are constant changes of temperature 
going on in the sewer, causing expansion and contraction of the 
alr. Under certain conditions the temperature of a sewer has 
been raised from 60° to 110° by hot water being passed in from 
factories. The force generated by this increase of temperature 
can be calculated by formule as the pressure is inversely as the 
space occupied. Let V=original volume of the air of the sewer 
at its normal temperature. V*=volume of air after its increase 
of temperature. P the original pressure = 14°6 tbs. per square 
inch, ora column of water thirty-four feet high. P' the pressure 
after the increase of temperature, we have as V: V1:: P:: P} 
in this case the volume of air at 60° is 1057°1 cubic inch, and at 
110° 1159-2 cubic inches, the normal pressure being equal to 34 
feet head, we have 1059-1 : 1159:2: : 34 ft. : 37°28 a difference of 
head of 3:28 feet of water, equal to 1-41 Ib. on the square inch, a 
force no trap at present in use could withstand. It is plain that 
if no outlet exists for the escape of the air under pressure the 
seal of the nearest water traps would be broken and the house 
placed in direct communication with the sewer, hence the necessity 
for ample ventilation. 
Influx of water into sewers.—The influx of water into the sewers 
is also a source of danger in an unventilated system where the 
sewers are on the combined system, in this case the area occupied 
by the flow of daily sewage even at its maximum is small in com. 
parison with the whole area. A sewer running half full is sud- 
denly charged with water up to the three-quarter flow line, it is 
