Withy. — On Sanitation and Ventilation. 461 



these pipes was carefully trapped it was supposed that 

 no return of foul air could take place, and so every one 

 slept in peace. The important condition of the problem, 

 which had so far been overlooked in making the arrange- 

 ment in this way, was the fact that the pipes were 

 not constantly full of water but of air. From its light- 

 ness, the air necessarily fills all the spaces from trap to trap, 

 whilst the water flows down to the lowest level which it can 

 find. The air, of course, becomes polluted by the frequent 

 passage of discharges, and by constant contact with the dirty 

 pipe. " Evil communications corrupt good manners." What- 

 ever disease-germs may be present will thus be carried about, 

 not only by foul water, but by the air which it has contami- 

 nated. It soon became evident to some who studied the sub- 

 ject that w T hen the pipes were thus charged, if any water was 

 poured into one of them, it would, owing to its superior gravity, 

 fall through the air, and either cause the immediate displace- 

 ment of an equal bulk of it, or produce a temporary increase 

 of pressure in the pipes. This pressure would be gradually 

 relieved by the escape of air and water until the normal con- 

 dition was resumed. Now, the all-important question was, 

 " In what direction will the immediate displacement or gra- 

 dual escape of this air take place?" If it went downwards 

 with the discharged water, and passed through the several 

 traps into the drain or cesspit, all might be comparatively well. 

 But obviously this could not take place. In such a race the 

 heaviest fluid must necessarily win the race, and, in its de- 

 scent, must force upicards most of the air which was stationary 

 in the pipes. At the best, the water could only force before 

 it, or carry along with itself, a small portion of the air, leaving 

 the greater part to reascend the pipes and force its way, until 

 the normal condition of pressure was restored, through one or 

 other of the traps. Nor is this forcing of its way through the 

 water in a trap, which some from want of experience may say 

 is impossible, its only way of escape into the house. This air- 

 tight system of pipes involves other dangers, one of which is 

 that the water may be siphoned out of any trap by the sudden 

 discharge of a large quantity of w y ater, either through itself, or 

 down some other pipe of the same system. If such a body of 

 water rushes impetuously down a nearly vertical pipe, tem- 

 porarily forcing onward some of the air, it must produce a 

 partial vacuum behind itself. The inrush of air from the 

 house to fill this vacuum will often be strong enough to carry 

 with it so much of the water which was lying in the trap as 

 to leave a free passage for the subsequent ascent of the foul 

 air. When this ascent takes place, if it happens that the waste- 

 plug or valve has not been returned to its place immediately 

 after the discharge has passed away, the foul air will slowly 



