of 1863, the loss of water by evaporation and absorption was only 83,674,843 cubic feet, represented by 5.124 out of 39-3 inches of rain-fall, or only 13-17 per cent. of the whole rainfall.

In the four months commencing with April and ending with July 1864, you will notice the gradual increase in the loss, beginning at 43 and ending with 82-25 per cent. in July, the maximum rate of loss having then been attained.

August was a most exceptional month. The absorption and evaporation are represented by 28 inches of rain. But the rain for the whole month having been only 65, and the land being already in such a state of drought as to have ceased, or almost ceased, to give off water except from the springs, the rate per cent. by absorption and evaporation falls to 43.

One reservoir was already empty, and the area of water in the other being greatly diminished, the evaporation from the water surface was necessarily diminished in similar proportion. The supply to the town being at the same time curtailed, the produce of the springs bore a larger proportion to the whole supply than in any of the preceding or subsequent months; and to these concurrent causes, in so far as I am able to judge, the small percentage of absorption and evaporation is due.

Taking the six months of April, May, June, July, August, and September, the results are still more curious, and show, in a striking manner, the intensity of the drought during the whole of that period.

The aggregate rain-fall for these six months was only 10.89 inches, representing 176,738,311 cubic feet of water, out of which the loss by absorption and evaporation amounted to the enormous quantity of 122,737,352 cubic feet, representing 7.489 inches of rain, out of a rain-fall of only 10-89 inches, or a loss of 69.44 per cent.

I have only to state, in conclusion, that I greatly doubt whether there is any instance of a drought of similar intensity ever having occurred in that locality. During the whole seven months of its continuance, besides fulfilling their statutory obligations to the mill-owners on the Esk,

the Water Company were able to maintain a supply to the inhabitants of Edinburgh, Leith, and Portobello, which only during six days, and these in the month of October, fell so low as 451 cubic feet per minute, or 20-23 gallons per head per day, to a population of 200,000 persons.

On a Proposed Safety Embankment for Reservoirs. By ROBERT AYTOUN, F.R.S.S.A.*

The fearful loss of life which has taken place recently, on two separate occasions, by the bursting of artificial reservoirs, coupled with the fact that so many of our towns are dependent for their supply of water upon reservoirs with the like liability to burst, must be my excuse for bringing before the Society a plan of embankment which I believe to be much safer than that at present in use.

The embankment at present in use is very badly formed. The puddle is built in the form of an upright wall, running along the whole length, and rising to the top of the embankment. It is supported on either side by large banks of earth, sloping two and a-half or three horizontal to one perpendicular. The front bank being completely soaked with water, is incapable of offering any effectual resistance to the pressure of the water in the reservoir.. In fact, it is barely able to preserve its own form, and to prevent itself from sliding down into the water, and leaving the puddle-wall unsupported. This is the reason of the apparent anomaly of giving to this bank, which has nothing to do but support itself and one side of the puddle-wall, greater dimensions and greater breadth of base than to the outer bank, which has to resist the whole pressure of the water in the reservoir. The outer bank, being kept dry by the puddle-wall, should be capable, by the friction of its dry particles among each other, as well as by their mutual adhesion, of opposing a determinate resistance to the thrust of the water in the reservoir. This resistance may be safely calculated at one-third of the

* Read before the Society on 13th February 1865, and awarded the special thanks of the Society.

weight of the materials of which the bank is composed. The outer bank, then, is the only part of the embankment of the present day which is utilised in confining the waters of the reservoir and preventing them from bursting forth.

In the embankment which I propose, the puddle is to be laid down on the surface of the inner slope (see woodcut on preceding page). By this simple change in the place of the puddle, the inner bank is kept dry as well as the outer, and the weight of both banks-that is to say, of the whole embankment is brought to bear against the pressure of water in the reservoir. This at once doubles the strength of embankment formed of a given quantity of materials, and would of itself warrant the employment of the word "safety" to this form of embankment.

But it has another claim, which, if substantiated, fairly entitles it to that epithet. The pressure of water in the reservoir comes, not upon the earth which it permeates, but upon the puddle which keeps it back. But the pressure of a liquid on a surface acts at right angles to that surface. Consequently, the pressure of the water of the reservoir meeting with the upright wall of puddle, of an embankment of the present form, acts against it with its full effect, in tending to push the whole embankment down the valley.

Not so with the proposed embankment; for the puddle lying at a gentle slope-say of one perpendicular to three horizontal-the pressure of the water acts at right angles thereto, and is resolved into a horizontal thrust and a vertical pressure, which are to each other in the proportion of one to three. This vertical pressure of three creates a frictional resistance to motion of one-third of itself, or one which is exactly equal to the horizontal thrust. It thus appears that the pressure exerted by the water of the reservoir may be so modified by a sloping impervious bank, that the horizontal thrust against the bank is completely neutralised by the additional friction which the pressure itself creates; and that the paradoxical statement may be hazarded, that the pressure of the water has in this case no tendency to push down the embankment, and consequently that the weight of the embankment has a complete sinecure in that respect.

It is not necessary to enter into all the details which require to be attended to in laying down the puddle on the slope. In my opinion, it should be covered with a considerable thickness of macadamised stones, to prevent the entrance of vermin and the wash of the water, and then be well. pitched with stone. The foundation of the puddle slope may be secured in the usual way by digging a trench down to an impervious stratum, and filling it with puddle; or what is simpler, cheaper, and better, the puddle may be continued at the foot of the slope, by laying it on the natural surface of the ground, forming a margin of puddle of sufficient breadth to prevent the passage of the water of the reservoir below the puddle-slope.

One very great advantage of laying the puddle on the slope is the possibility of repairing a leakage. By lowering the water, the puddle, pipes, &c., can be examined and repaired, and this can be done with little labour or expense. This is not the case when the puddle is in the middle of the embankment, in which case the repair of a leakage is all but impossible, and would be attended with great expense.

With the sloping puddle, also, the largest pipes and conduits that may be desired for drawing off the water may be. -employed, partly because of the ease with which any leakage at their junction with the puddle may be repaired, and partly because they are not exposed at this critical place-their junction with the puddle-to the enormous pressure which so frequently fractures them when the puddle is in the middle of the embankment. The power of using pipes or conduits of larger size than what is necessary for the ordinary supply is of great advantage. For by opening the valve, which should always be placed at the reservoir side of the embankment, no more than is sufficient for the ordinary supply, the pipe or conduit will be only partially filled, and the pressure inside reduced to that of the atmosphere. Any leakage, therefore, from it will be trifling. In fact, so far from the pipe or conduit injuring the embankment by leakage, it will act the part of a drain, and carry off any water which may have found its way into it otherwise. At least this will be the case if the pipe or conduit be so large,