The fourth essential, as stated above, calls for a suitable trap, placed as near as possible under every fixture.

As regards this point I cannot agree with the views of Prof. Osborne Reynolds of Owens College, Manchester. In his otherwise excellent little book, "Sewer Gas and how to keep it out of Houses," after explaining the necessity of a disconnecting trap on the main drain, and giving particulars about its construction, he continues: "There will then be no need to have traps within the house."

Traps under fixtures become a necessity, as much of the so-called "sewer gas " is actually generated in the drain and soil pipes of the house. Even the waste from a wash bowl becomes coated in time with a soapy slime, emitting bad odors. The trap on the main drain would offer no protection against the foul gases derived from organic matter decomposing within the pipes. We thus see that, while some advocate the trap on main drain, but no traps under fixtures, others leave out the main trap, but trap the outlets of all fixtures. In my opinion, both the trap on main drain and those under fixtures are necessary.

Traps should be located as close as possible to fixtures, in order to reduce the length of waste pipe on the house side of the trap, which is liable to become foul with long use. Probably the best material for traps is lead, as this permits of making a good joint with the lead waste pipes. As Mr. Hellyer has truly pointed out, the junction of the trap with the waste pipe is of far more importance than its junction with the fitting, because the former is on the sewer side of the trap, and, unless properly made, would afford a passage for gases from the waste pipe system into the rooms.

Whatever kind of trap may be used under fittings (and there is an endless number of such patented devices), it is of the greatest importance that the trap should be self-cleansing; for this reason traps with square corners or large spaces, liable to accumulate dirty matter, are objectionable. Much depends on a proper size of traps for waste pipes: the smaller the trap the better will it be washed clean. As a good rule I would recommend to choose a trap a quarter or half an inch smaller than the diameter of the waste pipe, to which it is attached. The flashing stream is thus concentrated, and its scouring power increased within the trap, while on the other hand a trap an inch larger than the waste pipe is sure to fill up in time with sediment.

The following will serve as a guide:

Traps under water closets with, 4 in. soil pipe should be 3 1/2 in. to 4 in. diameter.

Traps under wash basins with 1 1/4 in. to 1 1/2 in. waste pipe should be 1 in. to 1 1/4 in. diameter.

Traps under bath and foot tubs with 1 1/2 in. waste pipe should be 1 1/4 in. diameter.

Traps under laundry tubs with 1 1/2 in. to 2 in. waste pipe should be 1 1/4 in. to l1/2 in. diameter.

Traps under sinks with 1 1/2 in. to 2 in. waste pipe should be 1 1/4 in. to 1 1/2 in. diameter.

Traps under slop sinks with 2 in. to 3 in. waste pipes should be 1 1/2 in. to 2 in. diameter.

As regards the proper dip of traps I would say that traps under those fittings which receive solids (water closets) should not have a greater dip than 1 1/2 to 2 inches, because otherwise the solids are not readily removed, and lodge in the trap. For traps of minor wastes a larger dip or " water seal" is advantageous, as affording a protection against loss of seal through evaporation, siphonage or back pressure.

Traps may be classified according to the means used for the exclusion of gases into:

1. Water-seal traps.

2. Mechanical traps.

The characteristic of all water-seal traps is that they have in their lowest part a bulk of water divided by a dip in the pipe, so as to stand on the house side as well as on the sewer side one or several inches higher than the lowest point of the dip, thus making a seal which, under ordinary circumstances, prevents the passage of gases.

The traps of the second class have, in addition to the water-seal, a mechanical contrivance such as floats, balls, valves, flaps, etc, to exclude sewer gas.

Of water-seal traps I mention the bell trap, Antill's trap, the old fashioned D-trap, the bottle or round trap, Adee's trap, the Climax trap, the common S-trap, P-fcrap and three quarter S-trap. There is in endless variety of mechanical traps, amongst which I mention Bower's trap, Cudell's trap, Garland's trap, Budian's trap, Waring's check valve, Nicholson's mercury seal trap, and others (see Fig.

2.)

The bell trap A is objectionable on account of insufficient water seal and improper shape. It is frequently found at the outlet of sinks and yard gullies, and being in its upper part a movable strainer, it is often lifted by servants or thoughtless persons, and the gases from the drain pipe thus enter the house freely.

Antill's trap B avoids this defect, having a fixed strainer, but is objectionable on account of shape and small water-seal.

The D-trap C and the bottle trap D constitute small cesspools; they violate the principle that a trap ought to be self-cleansing. The D-trap accumulates dirt and grease in the upper corner, which receives no scouring from the water passing through the trap; and the bottle trap very often chokes up as shown at E. A round trap of improved shape is shown at F, which may keep cleaner on account of its round bottom.

Adee's trap G is little better in this respect, though it has this to recommend it that it is not so easily siphoned, having a large air space above the water, and a large body of water in the trap. This is also true of the round trap, when new and clean; when choked with grease as shown at E, it is as much liable to siphon-age as the S- trap.

The Climax trap, H, has a large dip and a round cup at its bottom, which is removable for cleaning purposes. Its resistance to siphonage is not greater than that of any of the other traps, or that of the common S-trap with same depth of water-seal.

The P-trap I, and Strap J, are shaped so as to be perfectly self-cleansing when adapted in size to their waste pipes. They are of uniform diameter throughout, have no nooks or corners to accumulate dirt. The old hand-made S-traps with seams have been superseded by lead traps cast in a mould such as the Du Bois traps. As regards cleanliness these traps are undoubtedly superior to all other traps of which I have knowledge. They cannot, however, be relied upon to exclude sewer gas, as their water-seal is frequently destroyed either by siphonage or by evaporation. They are shown in Fig. 2, with a vent pipe attached at the highest bend of the trap on the sewer side of the seal. The object of this vent pipe is to prevent siphonage, as will be explained hereafter.

Bower's trap is shown at K. This trap has a water-chamber into which the pipe from fitting enters at the center, and an outlet pipe on one side. The mouth of the inlet pipe is sealed by the water in the chamber, but in addition to this a floating ball of india-rubber in the water chamber is held tightly against the mouth of the inlet pipe, forming a seal, which, however, depends on the quantity of water in the chamber. The water, in passing through this trap, removes the ball from its seat and rotates the same, thus keeping it clean and free from matters adhering to it. An additional advantage of this trap lies in the ball, which, being compressible, allows the water in the chamber to freeze without danger of the bursting of the cup. Unless the soil pipe is extended full size through the roof this trap may have its water lowered by siphonage so much that the ball will drop from the mouth of the inlet pipe, but with proper ventilation of soil and waste pipes it forms an efficient trap for wash bowls, tubs and sinks, although it is not as self-cleansing as the common S-trap.

Waring's check-valve is shown at O. This valve forms a seal by its weight, and the seal is dependent upon the accuracy of the turned seat. Hair and particles of other matters may adhere to it and prevent a tight shutting of the valve.

Cudell's trap L and Buchan's trap M are constructed much upon the same principle, but have a heavy metallic ball instead of a conical shaped valve. This ball may keep cleaner by being revolved, but in this cast, as above, the tightness of the seal will depend upon the accuracy of turning the seat.

Nicholson's mercury-seal trap N has an inverted porcelain cup inside of its cylinder, the edge of which rests on mercury, forming a tight seal. The cup is lifted, at each discharge, by the force of the water entering at bottom of cylinder; after all water has passed from the basin the cup falls back in its place. This trap is generally made of earthenware with brass couplings; it is therefore a more expensive trap, but the mercury seal very efficiently prevents the entrance of sewer air, even if the water in the cylinder should be removed by siphonage or evaporation.