154 ON THE WATER IN THE CHALK 



time to come for its ever-growing population ; there can be no 

 doubt, that for the ^.r/r«-metropoHtan part of our county, we have 

 directly under our feet a practically inexhaustible supply. Thus, 

 our interest in the quality of that supply is direct, and anything 

 bearing upon it of immediate practical interest. But before 

 addressing myself to this question, I must say a few words more 

 as to the flow or motion of the water through the chalk. 



I said just now that if a well were sunk at this place into the 

 chalk, the water would rise up in it until it stood at about the 

 level of the sea. Now, the water in a well in the open chalk 

 country, say north of Watford, would stand at a level of some 250 

 feet or upwards above the sea level, and we know that if a free 

 and closed communication were opened (say by an iron water 

 main) between such a well and our well here, the water would rise 

 to the same level — or rather, the surface here, being only about 

 180 feet above the sea, would overflow. If, however, the pipe 

 were partially choked, and if it were also tapped to feed other 

 places at lower levels, the level to which it would rise here would 

 be certainly lowered. Now, this is what v/e must conceive really 

 happens in the chalk beneath our feet : the passage of the water is 

 obstructed by the solid chalk itself through which it can percolate 

 but slowly, and large volumes of water are carried off to feed the 

 lower springs which supply the Thames. Hence it is not surpris- 

 ing that the water in our wells does not rise to anything like the 

 level of the source whence it is derived. From a careful tabula- 

 tion of the heights to which the water has been found to rise in 

 numerous wells in the Thames Basin, Mr. Joseph Lucas, late of 

 the Geological Survey, has been able to lay down on the maps, 

 which I have here, and which he terms Hydrogeological maps,* 

 a series of lines, along each of which it may be expected that 

 water would rise, in wells sunk into the chalk, to the same level 

 above or below the O.D. Along one line, for instance, which 

 passes through Kensal Green, Kilburn (very near the site of this 

 building), the south part of Hampstead, Highgate, and a little 

 south of Hornsey and Tottenham, Mr. Lucas tells us the water 

 w^ould rest exactly at the level of the O.D. Proceeding north- 



* Published by Stanford, May, 187S. 



