THE UNDERGROUND WATERS OF NORTH-WEST YORKSHIRE. 39 
readings were taken every 15 minutes for many hours after the 
flush, and absolutely no rise occurred. On the other hand, as 
has alread}^ been stated, in the beginning of August and again 
later in the month, on which occasions the drought w^as at its 
maximum severity, the Cove, or at least the stream, at Malham 
Bridge rose in response to each flush. In the experiments in 
June, though no rise of the water-level took place at the Cove, 
traces of the ammonium sulphate introduced at the Tarn Water 
Sinks were found in the water issuing from the Cove eleven 
days later. These apparent anomalies appear to be susceptible 
of a fairly simple explanation, and one which throws much light 
on the movement of underground water. The upper limit of 
saturation of a pervious rock forms a somewhat irregular surface, 
whose altitude and slope vary according to (1) the freedom of 
escape of the water at various points, (2) the facility of percola- 
tion or flow in different directions, and (3) the interval w^hich 
has elapsed since the last absorption of rainfall. 
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1. The points of escape of underground water. There will 
be a general slope of the saturation planes so as to produce 
a series of slopes converging upon each of the points of escape. 
In the case of a well the slopes will form an inverted cone 
(sometimes called the "cone of exhaustion") during pumping or in 
the recovery from pumping (Fig. 1). When the escape is by a 
spring, the slopes will generally assume the form of a half cone 
