In the event of the presence in one apartment of a contagious disease, it is possible in the use of the system of Fig. B to communicate the disease to the inmates of other apartments in the building.

This would be especially true of apartments the water closets of which backed up to opposite sides of the same partition. In the same way, in the use of the system of Fig. B, conversation and other sounds may be carried from the toilet room of one apartment into the toilet rooms of other apartments. The separate system of local vents suffers from none of these objectionable features, and although certainly somewhat more expensive to install, the additional outlay should not be considered if the matter of freedom from the evils mentioned is to be secured. The local vent from a single water closet should never be less than 2 in. in diameter. When two, three, or four vents enter a main line of local vent, the main vent should not be less than 3 in. in diameter.

These are the sizes ordinarily used in the local-vent system, and are the sizes generally specified in plumbing ordinances, but are not strictly in accord with the principles that should be followed in securing a perfect system of ventilation.

Providing that a 2-in. pipe is of sufficient size to thoroughly ventilate a single water closet, at the point where the second vent enters, the pipe should be enlarged so that its area shall be equal to the combined area of the two vents which it supplies. When the third vent enters it, the size should be such that its area will be equal to the combined areas of the three branch vents. This gradation in the size of the main local-vent pipe is necessary if each water closet is to receive its full amount of ventilation, that is, if each water closet is to be ventilated as it would be if its individual 2-in. local vent were able to perform its full duties. The area of a 2-in. pipe is 3.14 sq. in.; of two 2-in. pipes, 6.28 sq. in.; and of three 2-in. pipes, 9.42 sq. in. The area of a 3-in. pipe is 7 in., and it will therefore be seen that while a 3-in. pipe is sufficiently large to provide for two 2-in. vents, it is not large enough to provide for a larger number.

The main, in order to properly provide for three fixtures, should be 3 1/2 in. in diameter, and 4 in. for four fixtures. While 2-in. local vents to the several water closets will accomplish good work, single vents of 2 1/2 in. diameter will be found to do better work. When this size is used, it will be found that two water closets will require a main vent 3 1/2 in. in diameter, and three water closets, 4 1/2 in. in diameter. This shows an increase in the main local vent of one inch in diameter for each additional water closet, but after the third fixture has been added the increase in the size of the main need not be so great. Water closets on which the local vent is to be connected should be provided with a spud, which may be on the right or left-hand side, as may be desired. As the local vent has no connection with the drainage system or with the trap-vent system, it is not an essential feature that its joints should be gas tight. For local vents either copper or galvanized sheet-iron pipe is used. Where the vent is exposed to view, and neat-looking work is desired, the copper pipe may be nickel plated. All changes in direction, reduction or increase in size of local vents should be made with long ells, reducers and Ys.

Y-branches and 45-degree bends are preferable to tees, as they make the course of the air currents more easily taken, and thus improve the draft.

The local vent should pitch upward throughout its course, in order to facilitate the work of the vent as much as possible. Heated air naturally rises, and therefore it is always poor practice in running pipes to convey such air in any other way than pitching upward toward the point of delivery. For the sake of convenience local vents are often bent downward to avoid some obstruction, and then carried upward again, a very poor practice when it can by any means be avoided.

Main local vents connected to a heated flue should not have an area exceeding one tenth the area of the flue itself. Local-vent connections with heated flues should always be made at points above the highest opening into the flue. If made below, the foul odors carried in the local-vent pipe may escape into the rooms with which flue openings communicate.

Care should be taken in making the proper chimney connection for local vents. An excellent method is to use copper pipe for connection into the chimney, the local vent lines being connected to the pipe. A cast-iron ferrule may also be used for the purpose, but galvanized sheet iron should not be used, as the soot of the chimney is liable to destroy it after a time.

The chimney connection may be run straight into the chimney, or it may be turned upward, an objection to the latter method being the danger of the collection in the pipe of falling soot.

When so constructed, it is good practice to provide a cleanout at the outer end of the chimney connection, for use in clearing any obstruction.

The pointing downward of the pipe by means of a bend inside the chimney obviates trouble from the soot, but results in checking the draft.

When the chimney connection is run straight into the chimney it should project inside only slightly, as unnecessary obstruction of the flue space is undesirable.

The work which has thus far been described and illustrated relates chiefly to the application of the local vent to residences, dwelling houses, apartment houses of ordinary size, etc.

For larger work more extensive methods are necessary, such as the use of large piping, and the mechanical supply of fresh air and exhausting of foul air.

In the case of public toilet rooms, underground comfort stations, etc., means of ventilation on a large scale are extremely necessary, as the use of such rooms would otherwise result in a public nuisance.

The difference to be noted in the atmosphere of public toilet rooms of hotels, for instance, which are provided with poor light and no ventilation, is great in comparison with the atmosphere of many of our modern, well-appointed toilet rooms of hotels, etc.

It has become a matter of good business to make special effort and outlay in securing proper ventilation for toilet rooms of public buildings, for the public has become educated to the point where they will patronize only those establishments that look after these points.

On the larger work it often becomes necessary to secure greater motive power for ventilating purposes than the heated flue is able to furnish.

For this purpose fans are largely employed, connected as shown in Fig. D. Usually an exhaust fan is used to withdraw the foul air, and another fan to supply fresh air to the fresh-air ducts.

This class of work will be taken up again in connection with the subject of public toilet rooms, as also the local venting of urinals.