OF THE IfEIGHBOURHOOD OF WATFOED. 129 



wider and entire district. The Chalk is often spoken of as a water- 

 bearing stratum. That it is the great source of water to the 

 Thames and its tributaries is beyond all doubt, and this, because 

 it receives the rainfall on an extended siu'face, and a certain 

 portion of that rainfall sinking into the earth forms an extensive 

 reservoir, whence the Thames and most of its tributaries derive 

 their waters. That the subterranean water in the Chalk stratum 

 is due to the rainfall on its surface needs no argument. It has 

 been calculated that after the summer months, when generally 

 there is no percolation of water to the subterranean reservoir, it 

 requires at least three inches of continuous rainfall to replace the 

 evaporation from the soil. When the soil is saturated, a consider- 

 able per-centage sinks into the earth and an accumulation of water 

 takes place ; and as the water moves, by natural gravitation, 

 towards its vent or outfall — the rivers which run in the lowest 

 valleys — in its passage thither it encounters a resistance which 

 so retards its progress that it assumes an inclined surface, 

 which is the balance between this resistance in the medium in 

 which the water flows, and the hydrostatic force by which its 

 progress towards its vent is quickened. This inclination of surface 

 varies with the presence or absence of cracks, fissures, and orifices, 

 through which alone the water in the Chalk can flow. The distance 

 from the outfall regulates the amount of storage in this subter- 

 ranean reservoir in the Chalk. The whole average inclination of 

 the surface of the water, from the Chiltern Hills to the Colne, is 

 13 feet in the mile. Besides this, which may be called the longi- 

 tudinal, there is a lateral adjustment of inclination of sui'face 

 rising from the river on either side to the centre between two 

 rivers, flowing in parallel lines towards the south. Thus the river 

 Gade, at or about Cassio Bridge, and the River Colne, at some 

 points between Watford Mill and the site of the old Bushey Mill, 

 will be found on a level. Draw a line connecting these two points, 

 and take the four cross-roads as a centre between them, and it will 

 be found that the water there is never less than 10 feet above the 

 rivers, and that this height decreases progressively towards the 

 rivers, and, moreover, that in very wet seasons there will be an 

 additional rise of about 10 feet at the top of Watford, varying with 

 the amount of rainfall percolating to the water-level in the Chalk. 

 Take a third line following the course of the High Street of 

 Watford, and this will be found to agree with the other Hues. 

 At one time the Chalk under Watford was perforated with wells ; 

 they are now mostly, if not all, filled up, but the measurements of 

 the varying level of the waters have been retained, as well as 

 those from the top of Watford to Cassio Bridge. The way in 

 which the water-level in the Chalk maintains its regularity, and 

 its ratio of rise and fall, may be deemed an interesting phenomenon 

 of natural geological hydraulics. 



It is by the replenishment and exhaustion of the rain-water 

 sinking into the Chalk that the rivers are fed and maintain their 

 volume, the water in wells following the same natural law. There 



VOL. I. — PT. V, 10 



