The most marked example of interference with which I have met was in the case of two ancient wells at El Dêr el Ghennîma, situated only 88 metres apart, on the crest of an anticlinal fold running north and south. These wells had been sanded-up for centuries, but were recently taken in hand and cleaned out. The difference of level in the outlets is 2·07 metres, the higher well being 34½ metres in depth, the lower 41 metres. The opening or closing of the lower well produces an almost instantaneous effect on the higher, the difference in flow of the latter within thirty seconds amounting to as much as 11 per cent. A great many observations were made, but the following are sufficient to show the rates of decrease and increase: Gallons per Minute. Upper well flowed 13·2 when lower well was open (flow 50·3 g.p.m.). „ „ 20·5 „ „ had been closed 10 minutes. „ „ 23·7 „ „ „ „ 20 „ „ „ 26·3 „ „ „ „ 30 „ „ „ 39·4 „ „ „ „ 24 hours. „ „ 35·6 „ „ had been open 30 seconds. „ „ 32·5 „ „ „ „ 2 minutes. „ „ 26·7 „ „ „ „ 7 „ „ „ 24·9 „ „ „ „ 9 „ „ „ 23·4 „ „ „ „ 16½ „ „ „ 19·7 „ „ „ „ 45 „ The closing down of the lower well is thus seen to have influenced the discharge of the upper to the extent of 100 per cent, in the short space of thirty minutes, while the flow was trebled in twenty-four hours. On opening the lower well the discharge of the upper fell to within 50 per cent. of its normal within forty-five minutes.