ratio of its depth below the surface. Let h the depth of water in a reservoir; A = the area in square feet of a vertical

section of the embankment of the depth h; then the lateral pressure upon the embankment in a horizontal direction is, in lbs.,

P = h× 62 × A = 31.25 A. h,

a cubic foot of water weighing 62 lbs.

Or, generally, the whole pressure of water upon a submerged plane surface is equivalent to the area of the surface, multiplied by the weight per cubic unit of the fluid, and by the head of water measured from the centre of gravity of the submerged surface. That is, for water

P = 62. A1. h.

Where h1 the depth of the centre of gravity below the level of the water in feet; A, the area of the surface in feet; P = the pressure on the surface in lbs.

Fig. 87.

And the whole pressure in any one direction is equal to the area of a section of the fluid vertical to that direction, multiplied by the weight of a cubic unit of the fluid and by the distance of the centre of gravity of the section from the level of the water.

8 = the slope a d; h = the height af; k the breadth f d, all in feet;

p the centre of gravity of a b c d. Then the distance of the centre of gravity of a b c d from the level of the water, o p, is equal to h; and

Fig. 88.

the distance of the centre of gravity of the plane a bef from the level of the water is also h.

Therefore, the whole pressure upon

a b c d = lsh × 62.5 = 31.25 l 8 h.

The horizontal pressure against the embankment l.h2 x 62.5 = 31.25 l. h2.

=

The vertical pressure = l x k xhx 62.5

To the statistics given above of the rainfall and evaporation in this country it will be necessary to add some account of their amount in tropical climates, where the conditions are essentially different. In such climates for three-quarters of the year the rain never falls, and the whole quantity for the annual consumption falls during the remaining quarter. At the Bombay Water Works constructed by Mr. Conybeare, the annual rainfall is 124 inches, of which are assumed to be available for storage. The area draining into the basin is 3,948 acres, so that the supply is upwards of 6,600,000,000 gallons. The storage capacity of the reservior is 10,800,000,000 gallons, or 1,733,000,000 cubic feet.*

ΤΟ

At the Melbourne Water Works constructed under the direction of Mr. Matthew Bullock Jackson, the area of the reservoir when full is 1,303 acres, greatest depth 25 feet 6 inches, average depth 18 feet, and capacity 6,400,000,000 gallons. The area of the natural Catchwater basin is 4,650 acres, together with

* Minutes of Proceeding of Institute of Civil Engineers, vol. xvii. p. 560.

600 acres drained by a watercourse.

This area, however, may

be increased if a larger supply is necessary. This watercourse at the same time opens a connection with the River Plenty, through which flows the water drained from an extent of 40,000 acres of country. This watercourse is opened during the winter to fill the reservoir from this source. The following table gives the detail of the rainfall and evaporation observed by Mr. Jackson during the construction of the works :

TABLE, SHOWING THE AMOUNT OF SPONTANEOUS EVAPORATION AND RAINFALL FOR TWELVE MONTHS ENDING 31ST JANUARY 1858.

It is to be presumed that the evaporation given above, nearly three times the rainfall, is the evaporation from a surface of water such as that of the reservoir itself. The rain, however, is collected from a surface thirty-five times as great as that of the reservoir when at its maximum height.

Weirs or Dams, thrown across the beds of rivers, have always been employed in order to raise the head of water in the river bed, and to divert a portion of it for the purposes of the mill. We have now to consider how most economically to secure a sufficient fall, and to protect the dam from the destructive effects of floods.

There is hardly any department of engineering which re

quires more careful consideration than that of forming barriers to large quantities of moving water; and when the nature of rivers carrying off the drainage from a large area is considered, and the enormous power of suddenly accumulating floods, the nature of the resistance required from a dam may be easily conceived, and when all the care of the engineer has been exercised it nevertheless sometimes occurs that the torrents tear up and destroy in a night the work which was intended to perform the quiet industrial duties of a mill for ages, leaving, in place of the well-turned arch across the stream, only the horns of the abutments and an indistinguishable mass of rubbish mingled with the mountain debris of the flood.

Fig. 89.

Such is frequently the case with weir constructions, particularly those across the rapids of mountain torrents, and this not unfrequently causes the construction of a temporary dyke of boulder stones capable of withstanding the ordinary action of the river, and easily replaced when floods have caused its partial destruction. This

description of weir is carried diagonally across the stream at a (fig. 89), and being considerably longer than its breadth, forces part of the water into the conduit b, and passes the remainder over the top in a thin sheet, which does little or no damage to the banks below. In the above description of weir it seldom happens that much fall can be obtained, and they are

Another description of weir, which is generally employed on moderate-sized rivers, is the V form constructed across the bed of the river, as shown in fig. 90, in plan. The object of adopting this form of weir is to increase its resisting powers, and, by spreading the fall of water over a large surface, to diminish its destructive effects upon the apron below; the descending currents meeting in the angle of the V neutralise their effects on the foundations, and do less injury to the banks on either side. This weir is generally formed of piles (fig. 91), with an open

frame of timber, into which are inserted large boulder stones, forming a compact mass of boulder sheeting resting on gravel, and nearly impervious to water. Another weir, preferred to most others where timber is plentiful, is formed into a series of