89 



seepage water, that is, across the exposed end of the water table. 

 Such ditches may then be connected to one or more main ditches, if 

 necessary, and the water carried down the hillside parallel to the 

 seepage flow. Other recommendations to be noticed in drainage 

 work are the construction of V-shaped instead of flat-bottomed 

 ditches and the cutting of a V-shaped ditch about the width of a 

 shovel down the middle of a large ditch made to carry flood waters, 

 to prevent the formation of a series of shallow puddles in the dry 

 season. Vertical drainage, which is effected by means of wells sunk 

 vertically for the purpose of conducting water down through relatively 

 impervious soil into water-bearing sand or gravel, is usually advisable 

 only where surface drainage is very difficult or expensive. 



In oiling, no definite formula can be given for the oil to be used. 

 Straight kerosene is very effective, for it spreads rapidly and makes 

 a very thin film, but it also evaporates rapidly and is difficult to see 

 on the water, hence it is usually mixed with so-called crude oil in parts 

 varying from 3:1 to 1:3. The most satisfactory mixture is one 

 that is nearly black in colour and slightly thicker than kerosene in 

 consistency. In ordinary work the oil is generally applied by the 

 knapsack spray-can, which is furnished with an adjustable nozzle 

 capable of throwing a very fine spray. Drip-cans may be used with 

 advantage on small streams and ditches with a fairly good current. 

 A useful type of can may be made from an ordinary 5 or 10 gal. oil-can 

 having a spout near the top. The screw cap of this spout is perforated, 

 and a nail, packed round the basal end with waste, is inserted m the 

 hole and the can is laid on its side with the spout downwards. The 

 oil drips out along the nail and by tightening or loosening the latter 

 the flow can be regulated down to a few drops a minute. 



Oil-soaked waste or bags of oil-soaked saw^dust are often used as 

 a substitute for drip cans [see this Review, Ser. B, vii, p. 75], and a 

 new method of laying down oil by capillarity has recently been proposed 

 \loc. cit., p. 76]. This system enables constant oiling to be main- 

 tained whatever the direction of the wind, and is the method preferred 

 for permanent ground pools and drainage ditches, while it also enables 

 the labourers to devote most of their time to preventing obstructions, 

 particularly alga-^, which would interfere with the spread of the oil. 

 This method also effects substantial economy in the cost of labour, 

 oil and apparatus. 



Accessory measures are fish control and the use of larvicides, the 

 latter not being used at the present time to any great extent, except 

 where they constitute the waste products of chemical factories. Nitre 

 cake, which is the only one in anything like general use, is fairly efficient, 

 but of limited utility, being suitable only for old wells and abandoned 

 cisterns where there is no danger of poisoning persons or domestic 

 animals. 



Experiments are being conducted with a combination oil 

 larvicide in the form of creosote oils ; this is more lethal than kerosene 

 and may be effective without forming a complete film. So-called 

 " refined creosote " or commercial creosote, of a dark colour and a 

 consistency rather greater than that of kerosene, is applied in the form 

 of a fine mist-spray from a small hand-pump of | gal. capacity and 

 of the automatic type used for spraying disinfectants in a constant 

 atomised mist-spray. Such a mist will settle over the surface of the 



