— 161 — length involved an expenditure of £E.88.985 for 650 metres, or a cost of 136.9 milliemes per metre. The proportionate charge per feddan of irrigated land amounted to 291 milliemes.

Distributory canals to serve the individual " hoshes " of 100 feddans into which the unit has been demarcated, were constructed under supervision by the cultivators themselves, who are similarly responsible for their cleaning and maintenance, as also for the upkeep of the main canal.

Distribution of Water.—A prime object of management should be the reduction of avoidable losses of water between headworks and land. In larger systems it has been estimated that not less than a third of the headworks' supply is lost before reaching the outlet, and it is affirmed by competent authorities that such loss is capable of reduction by about 40 per cent., which is equivalent to a saving of about 14 per cent, of the headwork discharge. The greatest losses in such cases are incurred by the use of too numerous small watercourses, and the amalgamation of such channels into canals carrying between 2,500 and 5,000 m per diem largely increases efficiency. Such canals are capable of irrigating between 83 and 166 feddans in the Sudan, and have been universally adopted in laying out the experimental area. In the interests of malaria prevention and to facilitate regular cleaning, two such canals have, however, been pi-ovided on each " hosh," and are brought into use alternately for periods of four or five days. The intervals of disuse permit thorough drying, the destruction of mosquito larvae, and the limitation of weed-growth.

This arrangement is depicted in the attached plan.

The Gibb's Module.—Distribution to these smaller canals is effected through modules of the Gibb's pattern, as a great improvement on pipe-outlets. With the latter the water supply necessarily varies largely with fluctuations in the head of water in the main canal, whereas the module ensures a practically constant discharge.

In the Gibb's Module the water flows in a semi-circular path through a steel chamber, across which are fixed a number of vertical steel diaphragm plates. The lower edges of these plates are of such a shape, and they are fixed at such a height from the bottom of the chamber, as to allow a stream of just the correct required discharge of water to flow under them without interference. If, owing to an increase of head caused by a rise in the upstream water-level, the water tends to rise higher at the circumference of the chamber, then the water of the surface of the stream strikes against the diaphragm plates, and its centrifugal force being absorbed, it will fall in towards the centre. In this way the excess head that caused the additional rise of water at the circumference is used up by the fall back towards the centre.

It is hard to over-estimate the value of this contrivance as a means of facilitating control of the water supply in the interests of economy. With a determined duty of water and a known acreage of land to be irrigated, it suffices for all practical purposes to open a given module for a specified time and to M