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The wire is payed out until the weight is felt to rest on the bottom 
of the cesspool; it is then pulled up for about one foot and fastened to 
the nail. In this way the oiler will rest about eighteen inches from 
the bottom of the cesspool. (Fig. 3.) Two litres of oil are then 
thrown on to the surface of the water to kill the existing larvae, and 
the manhole door is closed. In the oil thrown on to the surface it 
has been found better to mix five per cent, of powdered resin, as this 
materially prevents evaporation. But resin must not be mixed with 
the oil inside the oiler, as it may clog the sand filter. 
The exudation of oil goes on at the rate of about 50 cubic 
centimetres a day for nearly two months. As oil comes out of 
spout A water enters the oiler through the spout B, and therefore it 
becomes heavier daily and consequently assumes a more horizontal 
position, until in about two months, when there is about three-quarters 
of a litre of oil left, the oil comes out of the spout B and the oiler is 
suddenly emptied of the mixture. (Fig. 4.) So that in the eighth 
week of the oiler’s existence in the cesspool there is a sudden and 
large ejection of oil on to the surface of the water. 
The reason we have designed this is that if the cesspool is a large 
one and the exudation of oil hardly sufficient, any larvae which may 
have evaded the oil towards the end of the two months will be 
destroyed. 
In ten weeks the oiler is removed and replaced by a new one. In 
Egypt the cost of each oiler complete without oil is sixpence. The 
apparatus has been given a fair trial, and has been found to be 
successful. It has saved a large amount of labour and oil, and will 
shortly be used extensively in Cairo and its suburbs. 
