confining ridges. The length of this basin is 1.95 miles, the water level 61 feet above datum, and the depth 30 feet or more. By lock No. 2, at the lower end of the second basin, the water level is again lowered 30 feet into a third basin extending for a distance of 125 miles to lock No. 1. By connecting this last lock with the

flanking hills by 10 small embankments, the lower section of the valley is partially flooded, and the excavation materially reduced thereby. Lock No. 1 drops the canal 31 feet to sea level. From this point to the harbour of Greytown, a distance of 9:30 miles, the canal traverses an alluvial sandy and swampy plain, but little elevated above the sea, with no features deserving of special mention.

A serious difficulty was anticipated in excavating the canal and forming the banks, in a solid and permanent fashion, through the low-lying region situated between the foot-hills and Greytown, known as "the Deseado swamp." This was cleared of its heavy timber, dredges were set to work, and the work was found easy. Banks

were thrown up for the canal, and an embankment for the railroad which is to traverse the swamp; and so solid was this material that it was found possible to abandon the original plan,-namely, that of building these banks upon "corduroy," to prevent their sinking out of sight.

The peculiarly solid character of the material is accounted for by the fact that, while the waters of the San Juan River are never discoloured by silt, or

sand, or mud (as mentioned elsewhere), the San Carlos stream, rising in Costa Rica and having a course of south-west to north-east, in times of flood is charged heavily with earthy material, in reality volcanic detritus. This discharges below the dam, through the Colorado Channel, into the Caribbean Sea. The perennial trade winds take it up and deposit it upon the outside of the Peninsula at Greytown.

THE LA FLOR DAM.

It

Numerous deep borings, made at the site of La Flor dam, showed the solid rock ledge to lie much deeper than the first auger operations indicated, and the original plans had, in consequence, to be materially modified. was intended to build this dam of "rock-fill," on the same principle adopted at Ochoa; but the great depths of soft earth overlying the rock ledge, in places 96 feet below the valley, renders that plan inapplicable, especially as here, unlike the San Juan, there is no large flow of water to assist in scouring the soft soil and in consolidating the "fill." A dam, with solid masonry core and earth slope, is now proposed, spanning the valley with a length of about 2,000 feet, an extreme depth, for 1,000 feet of its length, of 170 feet from crest to foundation of core (of which 70 feet will be above ground), and, in addition, core walls, aggregating about 500 feet in length, penetrating the abutment hills to the rock ledge. Locks Nos. 4 and 5 will also rest in this bed of rock, forming part of the dam abutment, and connecting with the core

G

wall at the western end. A waste weir, about 300 feet long, will be cut on the east side for the discharge of surplus water into the lower bed of the Grande. All this comprises a very important piece of work, but if good rock foundations and suitable material be available, as it is said they are, there is nothing to indicate serious engineering obstacles. Concrete will be used for the core walls and locks, the rock to be obtained from the (Western) Divide cut. The earth for the puddle fillings and embankments can be had from the canal excavation, or from the valley in the vicinity of the works.

DRAINAGE. LOCKAGE. LOCKS. CROSS SECTIONS.

In a country subject to a rainfall of 300 inches in the year, and more than 6 inches in twenty-four hours, the problem of drainage involves a contest with forces of nature whose destructive powers are a constant menace to engineering works, however careful and skilful their design and execution, and it is of the utmost importance, therefore, to reduce these forces to a minimum before the construction of works is commenced.

DRAINAGE (EASTERN DIVISION).

The large territory embraced between the ridge confining the basin to the south and the "lower route,”—the term used to designate the canal line formerly proposed on the left bank of the San Juan river,-is, by the

adoption of the "upper route," entirely eliminated from the problem of drainage, leaving only that portion of the watershed north of the ridge, from the Divide to the valley of the Machado, to be provided for.

The problem of disposing of the surplus waters in that portion of the route from the basin of the San Juan to the lower Deseado is one of importance. The flow of the San Juan at Ochoa at high flood in both the San Carlos and San Juan has been found by careful gauging to be 42,000 cubic feet per second. The river is known to have risen somewhat higher, however, and, as no gauging was made at the time, the above figures have been increased by 50 per cent., making the possible maximum flow 63,000 cubic feet per second, of which it is believed not less than two-thirds would probably come from the San Carlos, the upper San Juan not being subject to great alternations of flow. The combined basins of the San Francisco region have a watershed of about 65 square miles; and, allowing for a maximum rainfall of 12 inches in twenty-four hours-about twice the greatest rainfall-there will here result a possible discharge of 21,000 cubic feet per second. The watershed of the upper Deseado basin is about twelve square miles, which on the above basis will yield a discharge of, say,

A more thorough investigation of the discharge of the San Juan river is, in Major Dutton's opinion, desirable. As the handling and disposal of its waters will involve works of great magnitude and cost he suggests that a very careful measurement of its flow and fluctuations should be made, and a rating curve obtained, showing the ratio of discharge to height of water established by measurements maintained for two or three years.