138 JOHN EVANS — THE HERTFOEDSniEE BOTJKNE. 



year 1860, and probably in 1866. This is the more probable, as I 

 find that the Bourne near Croydon, in the Caterham Yalley, flowed 

 in 1841, 1852, 1866, and 1873, in which latter year it began to 

 flow about the middle of January. 



There are, you will observe, other Bournes in England which 

 are perhaps better known than this in Hertfordshire. They flow 

 generally in the same years as this, and the question arises, "\Miat 

 is the theoiy and meaning of these intermittent rivers, which seem 

 to flow in such a capricious manner — in some years flowing in 

 considerable force, and in other years being invisible ? In order to 

 understand the origin and theory of these bournes, you must con- 

 sider what is the origin and nature of all the streams which run 

 through a porous rock like the Chalk. 



Mr. Clutterbuck, in his lecture last month, gave you some 

 account of the generally accepted theoiy with regard to such 

 rivers. Taking, for instance, a valley with Chalk hills on each 

 side, and a river flowing down the valley, you will find that the 

 water which percolates through the Chalk goes down until it arrives 

 at a plane of permanent saturation. That plane of permanent 

 saturation is not level, but is inclined towards those places where 

 the water finds its way to the surface in the form of springs. It 

 varies at different times in the year, sometimes being inclined at 

 an angle of 20 feet and sometimes of 25 feet to the mile, and 

 at others being on a slope represented by not more than 1 2 feet to 

 the mile. The rain which falls during the winter mouths finds its 

 way through the Chalk, and raises the level of the subterranean 

 reservoir on account of the water coming in from above faster than 

 it can find its way out to the rivers; and although no rain may find 

 its way to the reservoir during the summer mouths, yet the water 

 already there is delivered out by springs, and the level gradually 

 falls, and you can tell by the variation in the wells what the fall 

 is. In a well at Studham, some miles from any stream, I have 

 found the level of the water in two successive years vary as much 

 as 70 feet. 



If we take the inclination of the rivers flowing past "Watford 

 and St. Albans, we find that they have an inclination of about 

 12 feet 6 inches to the mile in the lower portion of their course, 

 and 18 feet 6 inches in the upper portion. That shows wliat is 

 the amount of impediment to water passing thi-ough the Chalk in 

 a lateral direction. For, assuming that water could tind its way 

 through the Chalk at a less inclination than 12 feet 6 inches to the 

 mile, these rivers would cease to flow, or at all events you would 

 never see the streams upon the surface. The whole of the Chalk 

 being porous and pervious, iinlcss tlierc were a sufticiently saturated 

 bed beneath the river to hold the water up at an inclination at 

 least equal to that of the river, it would die in its bed and dis- 

 appear. The inclination of subterranean water passing tlirough 

 Chalk must therefore be at least 12 feet 6 inches or thereabouts to 

 the mile in this district. If, starting from the level of an existing 

 spring, you carried an artificial valley through a Chalk district at 



