The fifth requirement asks for a proper vent pipe for such traps under fixtures as are liable to be siphoned. This siphon-age constitutes in many cases a danger, but especially so with Straps. Traps may be siphoned under the following conditions:

1. Traps with an easy bend, on a rather steep line of waste pipe, and with small depth of seal, are liable to empty themselves by the momentum of the water rushing from the fitting through them. The air in the upper bend of the trap is expelled and replaced by water, which causes the trap to act as a siphon. When the fitting has discharged all its water, and air breaks the siphon, the water in its inner limb will mostly drop back into the trap, but in case of a small dip it would be insufficient to seal the trap. Unless a slow after-flush takes place the trap remains unsealed.

2. Traps under fixtures may be siphoned by a flow of water coming from another fitting on the same branch waste pipe.

3. Traps may be siphoned by a discharge - from a water closet, a tub, or from a pail of water from a slop sink - into the main soil pipe, to which the branch waste of the trap is connected.

To guard against the first danger the dip or water seal of the trap should be as great as possible; but, even then a special vent pipe will often be necessary, attached to the highest part of the bend in the trap on the sewer side of the water-seal, or else a mechanical trap should be used.

To guard against the second danger the trap of each fixture should be vented; wherever possible, each fixture should discharge independently into the soil pipe, thus reducing the danger from siphonage to cases 1 and 3.

The third danger from siphonage by a discharge into the main soil pipe, either above or below the point where the waste from the trap enters it, will in some cases be sufficiently prevented by the complete and thorough ventilation of the soil pipe. In many cases, however, the venting of the trap becomes necessary.

Where a number of water closets discharge into the same inclined branch of a soil pipe the air-vent to the water closet trap becomes necessary, especially so with water closets, discharging quickly a large body of water, such as the various patterns of the plunger closets (Zane, Dem-arest, Jennings) and some of the "washout" closets.

Where slop hoppers are trapped by an S-trap, this must be properly guarded against siphonage, as the trap is very likely to lose its seal from the momentum of the water rushing through it each time a pail of slops is quickly emptied into the sink.

The material most suitable for air pipes is lead, as such pipes are easily joined to lead traps. Sometimes wrought-iron tubing is used, and, since the vent pipe is not so much intended for carrying off foul gases [which office is performed by the vertical extension of all waste pipes through the roof] as to afford a passage to air in order to break the suction, they may be safely used. Care should be taken to lay these pipes with a slight inclination, in order to prevent accumulation of water from condensation in the pipes. Vent pipes for fixtures on different floors may be joined, if convenient, and may enter the soil pipe above the highest fixture. But it is preferable to run them to a main vent pipe of lead, or better, cast iron, which goes through the roof independently. Where this passes through the roof it must be enlarged to 4 inches diameter, as it might otherwise be obstructed by ice in winter time. It should not be covered at the top with any kind of ventilator. The size of the vent pipe should never be less than that of the trap, except for water closet traps, where it should be 2 inches in diameter, but in the case of two or more water closets it should be 3 inches and sometimes even larger from the point where the various vent pipes join

It is often not only costly but also in- . convenient to run vent pipes to the roof. There is also some danger that the vent pipes for traps under tubs, sinks and bowls may stop up with soapsuds or grease, in which case they would cease to act properly. The continuous current of air in the vent pipe, in passing over the water in the trap, will tend to increase its evaporation. Finally it becomes necessary in the case of high buildings, largely to increase the diameter of vent pipe in order to make up for the loss through friction necessarily occurring with long air pipes. Therefore, while I consider vent pipes for traps a necessary evil in many cases, I am inclined, in other cases, to prefer a good mechanical trap, which cannot be siphoned, provided the soil and waste pipe system has ample ventilation. Such mechanical trap may be used under sinks, tubs and bowls; but for water closets and slop hoppers (if without a strainer) the simple lead water seal trap with vent attached is the only safe device.