82 Report 8. A. A. Advancemkxt of .Sciknck. 



The weir A may be destroyed etich season, but the zaai dams 

 would probably serve for a good many. The dam making is chietiy 

 done with scrapers drawn by oxen, and comes out at a very low rate, 

 and besides, owing to the flatness of the lands a small amount of earth- 

 work will generally serve to flood very extensi\e areas. 



Perennial 7yru/rt#io?i.— Compared with the total area of cultivable 

 land in the country the area under perennial irrigation is very small. 

 This to a great extent follows as a natural consequence of the siuall 

 volume of perennial water llowing, and it may fairly be said that, 

 •except in the case of the Orange and Vaal rivers, very little, if any, 

 perennial water is allowed to waste in au}^ part oi the country where 

 it can profitably be utilised. 



The works constructed by the farmers for this class of irrigation 

 are usually of a simple nature, consisting of a weir and furrow, which, 

 however, are sometimes very substantially built. The weir may be of 

 stone and earth, of masonry, or conci"ete according to circumstances. 

 A weir much used by the farmers, simple and effecti^■e when well 

 made, is of dry stone and wire. It consists in reality of stone — usually 

 bouldei's collected in or about the river bed — built into the re([uired 

 foi-m and enclosed within an envelope of wire netting, wliich prevents 

 the displacement of the stones by the ri\er. Sometimes the wire 

 netting is made first, and laid down on the ri^er bed. The dr}' stone 

 dam is then built upon it, and the loose edges of the net are drawn 

 •over the top of the dam and fastened togetlier, thus completely enclos- 

 ing the rubble work. 



Another method is to lay down on the ri\er Vied single wires, 

 representing the warp of the net, and to work in the weft as the stone 

 work is built up. The former meth(jd generally gives neater work, 

 but probably costs rather more thn.n the latter. The wire used is 

 ordinary galvanised iron fencing wire. 8uch weirs are very effecti\e 

 up to heights of, say, 6 ft., particularly in rivers which carry much 

 silt, the silt very quickly filling the interstices between the stones and 

 rendering the work quite water-tight. Where a rock foiuidation is 

 obtainable very substantial rubble masonr}' weirs are often built, some- 

 times in Portland cement mortar, and sometimes in local lime. 



Another forni of weir which has given very good results consists 

 of a masonry or concrete core wall about 2 ft. thick, supported by dry 

 rock filling to a slope of IJ or 2 to 1. A weir of this nature about 

 1300 ft. long across the Vaal Hiver di\erts water into the Douglas 

 Canal, and a similar one about 1000 ft. long across the Breede Kiver 

 ■serves to supply the Robertson Canal -a very successful co-operative 

 irrigation work. 



Under the head of Perennial Irrigation falls also the method of irri- 

 gation by })umping from bore-holes by the aid of wind power. It will be 

 readily und(M'stood from the description given above of the alluvial 

 basins that much of the rain which falls will peicolate into the ground, 

 and springs more or less constant are frequently found in the river 

 beds, particulai'ly at the "poorts,' whei-e th(^ water is forced to the 

 surface by the underlying dolerite dyke. 



