In many districts, it is the usual practice to connect the rain-water pipes directly with the drains, under the assumption, possibly, that they will serve as ventilators. This is a very mischievous practice, and cannot be too strongly condemned as one that is fraught with much danger to the public health. When the pipe is most required as a ventilator, - that is, when the air of the drain is displaced by the sudden rush of the water caused by a rainfall, - the air forced to the rain-water pipes is met by the column of water descending from the eaves, and is forced out in another direction. Again, not only is the position of the heads of rain-water pipes often adjacent to windows, so that the drain-air readily communicates with the house, but by reason of defective joints, which are very common, escape also takes place there at frequent intervals.

These are amongst the mast common of the dangers and defects to be avoided in a drainage-system, apart from those met with in the sanitary appliances and fittings in the house. Those arising from the discharge of the sewage into pools willbe found described in the section dealing with "Sewage-Disposal"; and the disadvantages caused by a direct connection of the house-drain with the public sewer, arc dealt with in the subsequent chapter relating to "Ventilation, Disconnection, and Inspection".

One of the most important conditions to be complied with, in order to ensure a healthy habitation, is the necessity of having a system of immediate and perfect sewage-removal, in order that the air and soil may not be contaminated with excreta, nor the water-supply fouled by either direct or indirect contact with the sewage or its emanations. Where the sewage has been removed from the premises and delivered into the public sewer, this may be said to be accomplished. as it is then the duty of the Local Authority to convey it to some convenient spot, where it can be dealt with so as not to be a nuisance or injurious to health. In the absence of any system of public sewers, the sewage will require to be disposed of by one or other of the methods subsequently described in the section on "Sewage-Disposal".

In order that these conditions may be complied with, and the drainage-system perform its proper functions, its conception and design must be influenced by certain essential conditions, and upon the degree of perfection with which these principles are carried out depends the efficiency of the system. The essential conditions necessary to be observed are:-

1. Removal of Sewage. - The sewage must be rapidly removed from the building and its precincts, and the drains be "self-cleansing", so that there is no stagnation or collection of deposit in them.

2. Surface and Subsoil Water. - Wherever practicable, the surface and subsoil water must be conveyed away in separate drains.

3. Line of Drains. - All drains must be laid in straight lines from point to point, with true gradient-

4. Turning. - All turning must be done in "manholes" or "inspection-shafts", which must be placed at every change of direction or gradient.

5. Drains to terminate outside house. - Drains must not pass to the inside of houses, but end at an outside wall.

6. Drains under Houses. - Drains must never pass under buildings when it can possibly be avoided, and in towns where drains have to be carried through the house from back to front, special precautions must be taken.

7. Separation of Branches. - All important branches must be independent, and concentrated in a turning-chamber, and all branches be as short as practicable.

8. Inspection-chambers. - The drains must in all parts be readily accessible for the purposes of examination, testing, and cleansing.

9. Water-tightness. - The best materials must be used and skilled labour employed to ensure the drains being water-tight, so as to avoid pollution of the soil and air, and the admission of subsoil-water.

10. Size. - Drains must not be larger than necessary for the maximum duty they may have to perform, as the larger the drain in proportion to the quantity of sewage passing through it, the less is the power of the sewage to carry solid matters along, and vice versa.

11. Falls. - The falls of drains must, wherever possible, be sufficient to produce a self-cleansing velocity for the small amount of sewage that is usually discharged through them, apart from rainfall.

12. Depths. - Excessive and unnecessary depths must be avoided. Special means must be taken to effect the change of levels in drains.

13. Disconnection of Main Drains or Sewer. - The air-communication between the public sewer or cesspool, and the house-drain, must be severed by means of an intercepting trap fixed in a manhole or shaft.

14. Disconnection of Long Branches, - All exceptionally-long branches must, in like manner, be severed from the main house-drain.

15. Ventilation. - The drains must not terminate in a dead end, but be amply ventilated by openings so as to create an undiminished current of air through them.

16. Inlets to be trapped. - All inlets to drains must lie properly trapped, with the exception of those used for the purpose of ventilation.

17. Wastes to discharge in the open air. - All waste-pipes from sinks of every kind (except house-mauls slop-sinks), baths, lavatories, and other appliances from which foul matter is discharged, and all rain-water pipes, must deliver over gullies placed in accessible positions and connected with the drains outside the house.

18. Flushing. - If the available fall is not sufficient to produce a "self-cleaning" velocity in the drains, provision must be made for regular and frequent flushing. When the discharge of sewage into drains is only inter mittent, and not sufficient to prevent deposit in them, an automatic flushing tank should be fixed at the head.

19. Ma terials. - The materials must be the best of their respective kinds, and where the local conditions render it necessary, extra precautions must be taken in the selection of them, and all appliances of a special type should be tested in order to ascertain if they will satisfactorily perform the functions required of them, and only those should be selected that are the least likely to get out of order through the carelessness of a workman, or the neglect of those under whose care they are placed.