It may be desirable here to give the formule of the four different foreign authorities principally recognised in this country by engineers. I do not however consider either of them so reliable as that of Mr. Beardmore which I have adopted in the Table I have given.

Du Buat's formula reduced to English measure:

307 (√R−0·1)

V=

√S-L (√S+1·6)

V velocity in inches per second.

−0·3 (√R−0·1).

Rhydraulic mean depth= diameter in inches.

S slope or difference of level.

L=hyperbolic logarithm, and found by multiplying the common logarithm by 2.3026.

The quantities given by these formule were tested by actual discharges and the results were given at the Institution of Civil Engineers by Mr. Murray, C.E., in the following tabular statement. (See Minutes of Proceedings of the Institution of Civil Engineers, vol. xii. page 55.)

LXXXIII.-CIRCULAR AND EGG-SHAPED SEWERS COMPARED. An advantage is gained when the quantity of sewage varies from a small minimum to a large maximum by the adoption of egg-shaped sewers in the place of circular ones, the advantage consisting in the shape accommodating itself to the fluctuation of the flow. When the quantity is small the lesser diameter of the invert of the egg-shaped sewer affords a better scouring power than the larger diameter of an equivalent circular sewer, while the increased size of the upper part of the former conduit affords the requisite space for an increasing outflow.

In cases, however, where the sewage maintains a minimum flow equal to half the maximum flow so that the conduit is never less than half full, there is no advantage in an egg-shaped sewer over a circular one, while the latter is cheaper to construct and of greater strength.

The best form of an egg-shaped sewer is stated to be that given in Mr. Baldwin Latham's excellent work on Sanitary Engineering (p. 101). In this form the horizontal diameter is two-thirds of the vertical height, the radius describing the invert being one-fourth the horizontal diameter. The semicircle drawn on the horizontal diameter becomes the upper portion of the sewer,

By

while the segment drawn on the radius forms the invert. continuing the horizontal diameter half its length on each side, points are gained from which an arc may be struck for the sides to perfect the egg-shaped form. (See Drawing I., Fig. 3.) Mr. Hawksley adopts an egg-shaped sewer of his own form (see Drawing I., Fig. 3), which is very symmetrical, and in which instead of the vertical depth being to the horizontal diameter as 3 to 2 the proportions are as 5 to 4. To ascertain the horizontal diameter by which to construct an egg-shaped sewer equal in capacity to a circular sewer of any given size, Mr. Hawksley says, "Deduct its ninth part, and the remainder is the horizontal diameter of an equivalent oval sewer."

Thus, taking a 3 feet circular sewer as an instance, the horizontal diameter of an oval sewer of the same capacity will be 32 inches, while the vertical depth, which should be as 5 to 4 of the horizontal diameter, will be 40 inches.

I give here a Table showing the sizes, circumferences, and sectional areas of circular and egg-shaped sewers.

TABLE showing the Circumference and Sectional Area of Circular and Egg-shaped Sewers of different Sizes.

On Drawing IX., a number of egg-shaped and circular sewers are given in juxtaposition with some useful data for construction.

LXXXIV.-CONSTRUCTION OF Sewers and MATERIALS TO BE USED. Having spoken somewhat emphatically of the advantage of rendering all sewers water-tight as far as it is possible to do so, it will be my object now to point out the best means by which to arrive at that end. The difficulty, with a limited expenditure and under ordinary conditions, of constructing a perfectly water-tight sewer must be generally admitted, and it is for that reason that great care must be taken to attain that end.

The materials used for sewers consist of (1) bricks laid in cement or hydraulic lime mortar, (2) stoneware pipes, and (3) concrete, used separately, or in conjunction with bricks and pipes. Iron (4) also is used where the sewer has to be taken through unsound ground, under rivers, and in special cases, through closely inhabited districts.

(1.) Brick Sewers. In the construction of brick sewers it need hardly be said that bricks of the best quality should be used. I do not mean that they should necessarily be radiated bricks, but that they should be well burnt and well shaped, and possess adhesive qualities; for although the pressure of the outer earth upon the sewer may be greater than the internal pressure of the sewage outwards when the sewer is full, it is not the less necessary that the cement or hydraulic lime mortar in which the bricks are laid should adhere to the bricks where a water-tight condition of sewer is a paramount object. Ill-burnt and soft bricks should be most stringently rejected. Rough bricks, even if well-shaped and well-burnt, should not be used for the internal lining of sewers, as the suspended matters of the sewage will cling to them, and ultimately coat them with putrescible substances. The London stock brick forms a very good sample of a suitable brick for sewers. Some engineers prefer the Gault brick, but though it possesses a comparatively smooth surface, and therefore can be advantageously used for the inner lining, its lack of adhesiveness does not recommend it. The blue Staffordshire bricks and fireclay bricks glazed on one edge, form superior inverts, and being very hard, strong, and smooth will resist erosion. They are to be preferred to the glazed invert blocks which have been much recommended by some persons. (See Drawing IX.)

The cement or lime used for the mortar in the building of brick sewers should be selected with great care. The former should be Portland cement weighing from 110 lb. to 112 lb. to the striked bushel, and it should be used in the proportion of one of cement to one of clean washed sand. If lime be used at all, it should be the best hydraulic or blue lias lime. Roman cement, which sets more quickly than Portland cement, may be usefully applied as an inside