resultant pierces the base or any horizontal joint within certain defined limits. If the line of the resultant inter. sects any horizontal plane of the dam outside of these limits, stability is not assured.

The following conditions are indispensable for the stability of dams:

Ist. The courses of masonry must be incapable of slipping one over the other, and the wall incapable of sliding on its base.

2d. Neither the material employed nor the foundation must be required to bear too great a pressure.

The stones must not be laid in horizontal courses extending from front to rear, and binders should be freely used. The stability of all dams (or walls sustaining pressure) requires that there should be no continuous joints.

Earthen Dams. For reservoirs of moderate depth earthen dams are frequently used. Experience sanctions for these dimensions not less than ten feet on top, and a height of over sixty feet is considered risky by many engineers. Trautwine suggests that in properly constructed earthen dams "the top width should be equal to two feet plus twice the square root of the height in feet." The inner slope should be 21⁄2 (base) to 1 (height), and the outer slope 11⁄2 to 1. Flat inner slopes are most desirable, as they increase the stability of the structure and likewise prevent displacement of the pitching. In some instances the toes of the slopes abut against retaining walls in cement. The inner slopes should be carefully faced up to the top with dry rubble-stone pitching at least one and one-half feet deep.

The Pillarcitos reservoir, San Mateo County, has an earthen dam six hundred and forty feet long, twenty-six feet wide on top, and ninety-five feet high. The San Andreas dam is six hundred and forty feet long, twentyfive feet wide on top, and ninety-five feet high. The former has a slope of 234 (base) to I (height) on the inner, and 21⁄2 to I on the outer side. In the latter the inner

slope is 31⁄2 to 1, and the outer slope is 3 to 1. In both cases the puddle walls have been carried down respectively forty-six and forty-seven feet deeper than the base.

The materials selected for the embankment play a very important part. The best combination consists of gravel, sharp sand, and clay, properly proportioned, which give weight, cohesiveness, stability, and imperviousness. The weight of the wall must be opposed to the thrust, the height and length are determined quantities, and the thickness is the only remaining factor for adjustment.

Puddle Walls.-Engineers differ in opinion as to the value of puddle walls. They are designed to prevent leakage through or beneath the embankment and reach from the top to below the base. They should be from six to eight feet thick on top, increasing downwards by offsets at the rate of about one foot for every three or four in depth.

Where the embankment is composed of loose material and the water comes in contact with the clay puddle, it is advisable to enclose the puddle in concrete, or a watertight wall should intervene between the puddle and the reservoir.

A properly constructed embankment, with the inner slope and the bottom of the reservoir, especially near the toe, securely protected by means of puddle, concrete, or stone facing laid in cement, is considered by some engineers preferable to a puddle wall in the centre of the dam.

Shrinkage of Embankments.-The following are the approximate averages of the shrinkage of embank ments according to Trautwine (1882, p. 630):

Gravel or sand.

Clay......

Loam

Loose vegetable surface soil...

Puddle clay... . .

..8 per cent.

10 per cent. ..12 per cent.

15 per cent.

20 per cent.

* See Fanning, "Water-Supply Engineering," pp. 339–342.

Trautwine determined that one cubic yard of hard rock made on an average 1.7 cubic yards of embankment, or that one cubic yard of rock embankment required 0.5882 of a cubic yard in place. Also that a solid cubic yard when broken into fragments made 1.9 cubic yards of loose heap, 1 yards carelessly piled, 1.6 cubic yards carefully piled, 1.5 cubic yards very carelessly scabbled, or 1 cubic yards somewhat carefully scabbled.

Dams in California.-Among the most important dams built in California are: the Bowman dam, height one hundred feet, length four hundred and twenty-five

feet; three dams owned by the Milton Mining and Water Company, forming the English reservoir, the largest of these having a height of one hundred and thirty-one feet; the Fordyce, of the South Yuba Canal Company, five hundred and sixty-seven feet long and seventy-five feet high, catchment basin about forty square miles; the Eureka Lake dam of the Eureka Lake and Yuba Canal Company, length two hundred and fifty feet, height sixty-eight feet.

TABLE III.

Angles of Repose and Friction of Embankment Materials.*

* See "Treatise on Water-Supply Engineering," by J. T. Fanning, P. 345.

All the foregoing dams are built of dry rubble stone and faced with a water-tight lining of planks.

The Tuolumne County Water Company has built several timber crib dams, the largest of which is across the south fork of the Stanislaus River. This dam, which is three hundred feet long and sixty feet high, rests for its entire base on solid granite bed-rock. The cribs, constructed of round tamarack logs from two to three feet in diameter, are about eight feet square from log to log (ten feet centre to centre), and the timbers are pinned together with wooden treenails. The cribs have no rock filling.

The face is formed of flattened three-inch timber pinned with wooden treenails to the crib and calked with cedar bark. The flood water passes over the crest of the dam for the entire length. The water is drawn off by several gates, one above the other, placed on the inclined waterface. The dam was built in 1856. Its total cost did not exceed $40,000. Pine dams owned by this company, constructed on the same plan, have decayed, while cedar cribs are still in perfect order. The Spring Valley and Cherokee Company's Concow reservoir in Butte County is formed by two earthen dams, each about fifty-five feet in height; one of these, which is used as a waste, has its lower side built of heavy brush embedded in the earth.

THE BOWMAN RESERVOIR AND DAM.

This reservoir was designed for the supply of water during the dry season of the year to the gravel mines operated by the North Bloomfield Mining Company. It is located in a mountain valley, on Big Cañon Creek, a branch of the Yuba River.

It is fed from a gross catchment area of 28.94 square miles. Higher up on the same stream there are several other reservoirs owned by the Bloomfield and Eureka Lake companies, the upper one (Eureka Lake reservoir) holding 661,000,000 cubic feet of water. In ordinary seasons