202 THE NEW WORLD OF SCIENCE 



despite the dangers from waters both above and below, is suf- 

 ficient evidence of the excellent work done by Col. David and 

 his staff. 



Practically all rocks below a certain variable level known 

 as " the groundwater level," contain water so that tunnels, 

 dugouts or other excavations which go below that level will 

 be flooded. Hence it is necessary to know the distance from 

 the surface of the earth down to groundwater level all along a 

 battle front, and an important part of the work of the British 

 geologists was to prepare detailed maps showing just how deep 

 at any given place excavations could safely be carried, or how 

 deep wells would have to be driven to get plenty of water. In 

 the rainy season the groundwater level begins to rise, and as 

 the change takes place slowly there is a " lag," or the rising 

 continues for a time after the rains have ceased. An engineer 

 who drove a tunnel just above the groundwater level at the 

 end of the rainy season might well suppose that he was quite 

 safe, and yet have the tunnel flooded by a further rise of the 

 groundwater. Precisely this happened to a number of Ger- 

 man tunnels. Thanks to the skilful work of Col. David the 

 British were saved this misfortune ; for Col. David determined 

 not merely the groundwater level for a given time, but the 

 variations of the groundwater level as well ; and constructed 

 curves showing just how high the level would be at every time 

 of the year. He then directed the army engineers how to 

 locate their excavations so that they would never be flooded; 

 and British efficiency once more scored against the much 

 vaunted German efficiency. 



But it was not merely in locating excavations that the Brit- 

 ish geologists served their armies. They told the engineers 

 in advance what kind of rocks they would have to pass through 

 underground, and hence what kinds of tools and how much 

 timbering they would require. They kept a number of drill- 

 ing parties constantly busy making test borings to determine 

 with precision the exact thickness of every rock formation at 

 those places where the engineers planned underground work 



