The chief points to be considered in the selection of drain-pipes are strength, durability, tightness, smoothness, nature of the material, shape, and method of jointing.

1. Strength of drains. - At first sight it would appear as if there is no need for strength, but the ground in which the drain is laid may not be very solid, ami a slight subsidence may occur and thus bring a severe strain upon the drain; there is also the strain caused by the superimposed weight of the material replaced in the trench, which may be subsequently considerably increased if the level of the surface should be in any way altered. Again, there are certain districts, - such as in some colliery districts, where the workings are shallow and subsidence of the ground is continually going on, and in the brine-pumping districts, as the neighbourhoods of Northwich and Droitwich, - where sudden and severe strains are imposed upon the drains, rendering it extremely difficult to maintain them permanently sound. While in actual working there may be little risk of the drain being severely tested, it does sometimes occur that an exceptional stoppage will cause a whole pipe to be filled with water at a con-siderable pressure. While a house is being built, the drain may have to stand some rough treatment: or it may pass through a wall, and the weight or settle-ment of the wall may cause unequal pressure. The material of which the drain is made should be strong enough to withstand these various strains.

To test the strength of a drain it is necessary to try it in various ways. In order to ascertain its resistance to crushing, cut a hole to receive the socket, and bed the pipe horizontally and evenly, then pack and fill up with sand, and apply a weight uniformly along the top. For resistance to breaking, support the pipe on blocks and apply a weight at the centre. For internal pressure, use the ordinary hydraulic press, great care being exercised, in screwing up the press, to close the end of the pipe so as not to strain or fracture the material, and the pipe must be set plumb up and level, and the plates travel in parallel lines; if very accurate results are required, this is a very delicate operation.

I hiring the 1897 Sanitary Congress in Leeds a visit was paid to the works of the Leeds Fire-clay Company at Wort ley; and at Messrs. William Ingham & Son's branch, an ordinary 6-inch pi|>e, taken haphazard from stock, was tested for bunting, and did not collapse until a pressure of 150 lbs. on the square inch was reached. It is needless to say that such an extraordinary pressure would never be experienced in practice.

The weakest part of a pipe is at the closing up of the shoulder of the socket to the barrel, and at many works the sockets are put in by hand after the pipes are moulded; pipes thus made must necessarily l>e weaker than those made complete in one moulding.

Bearing in mind the strains to which pipes are subject in transit from the kiln to the trench, the proper place to apply a test for strength is at the spot where the pipes are delivered for laying.

2. Durability. - A drain will in all probability be alternately wet and dry, as the flow in it will be very variable; it will also be exposed to extremes of temperature, as the water may enter it at any temperature between freezing and boiling. On the outside, the chemical composition of the soil may affect tin-durability of the drain; while as regards the inside, the effect of the exceedingly-complex and ever-varying nature of the sewage cannot be ignored. The pipe has also to resist friction, and its inside surface must be sufficiently hard to do so. The bottom part, or "invert ". as it is technically called, necessarily has to bear the greatest strain from these wearing influences, the most destructive of which are the particles of sand and grit which are continuously borne along with the sewage. In the case of large sewers, it is the universal practice to construct the invert of harder materials than the other parts of the sewer, but in a house drain this is impossible; therefore all parts of the interior must be sufficiently hard. It is possible, however, to furnish the interior of a pipe with a hard skin that will also give smoothness, and, in the case of iron, one that will afford a protection against rust.

The resistance of a pipe to abrasion may be tested by applying a loaded small section of the pipe to a revolving grindstone kept wet and (lean, the diameter of the grindstone and the number of revolutions required to remove the glazing, or to wear away a definite thickness of the pipe, being recorded.

3. Tightness. - At one time little regard was paid to the tightness of a drain, either as regards the material itself or the method of construction. Fortunately, a very different standard of efficiency is now required. Whatever is put into a drain must be carried off without any loss on the way. in order that the subsoil under and around the pipe shall be kept free from contamination: and one of tie-necessities of modern sanitary construction is, that a drain must also be air-tight as well as water-tight, as the contents of a drain are liable to decompose and to give off dangerous gases.

The impermeability of a pipe may be tested by its capacity for absorbing water.

4. Smoothness. - It is evident that any want of internal smoothness would not only largely increase the tendency to deposit and the risk of actual choking, but the efficiency of the discharging capacity of the pipe would be reduced, by the roughness of the surface impeding the How and causing the solid particles in the sewage to adhere to the bottom; it must not be forgotten that, even in house drains, solid sul»tances of considerable size occasionally pass down, - such as pieces of floor-cloth and scrubbing-brushes, - and one of these, lodging against an accumulation of deposit, would quickly cause a complete stoppage of the drain. While it is not possible to have the interior of a drain as perfect as the interior of a long glass tube, still this is the ideal which the manufacturer has before his mind. Some materials, while otherwise suitable, have not this smooth surface, but can be made available by coating them with some other substance; this, however, is only efficient when the surface is so joined to the body as to be practically one with it. and continues to remain so.