HERTFORDSniRE NATUHAL HISTORY SOCIETT. lix 



The Eouvnc is flowing strongly this year, it flowed last year, and 

 also the year before, thus disproving the old idea that it only flows 

 once in seven years. The water is now seen to be bubbling up 

 from its fountain-head, Avhich is caused by the subterranean reser- 

 voir being full. The plane of permanent saturation — the surface 

 of this reservoir — is not level, but is inclined towards those places 

 where the water finds its way to the surface of the earth in the 

 form of springs. This plane varies at dift'erent times in the yoai", 

 sometimes being inclined at an angle of 20 feet, and sometimes 

 even of 25 feet to the mile, while sometimes the slope represented 

 is not more than 12 feet to the mile. The rivers flowing past 

 "Watford and St. Albans will be found to 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. This shows 

 what is the amount of impediment to water passing through the 

 chalk in a lateral direction ; for, assuming that water could find 

 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, the 

 streams would never be seen upon the surface. The whole of the 

 chalk being porous or pervious, unless there were a sufticiently 

 saturated bed beneath the river to hold the water up at an inclina- 

 tion at least equal to that of the river, it would sink into its bed 

 and disappear. The inclination of the subterranean water passing 

 through the chalk must therefore be at least 12 feet 6 inches to 

 the mile in this district. 



With regard now more particularly to this valley of the Bourne, 

 in certain years the subterranean water is raised in the body of the 

 hills, and the valley, cut at right angles or nearly so to the main 

 stream, intersects the general surface of the plane of saturation, 

 and the water appears on the surface of the ground at the bottom 

 of the valley. This theory of intermittent streams being due to 

 the intersection of the plane of saturation is well illustrated by 

 the Bourne being sometimes found running in one place, then dis- 

 appearing, and further on running again, and again disappearing. 

 The plane of saturation being more even than the surface of the 

 ground, where there is an elevation of the surface the water finds 

 its way underground instead of running on the surface. Owing 

 to the large amount of rain which has fallen in recent years, 

 having been above the average of a long period every year since 

 1875 to the present time, the Bourne has during this period very 

 frequently flowed. Since 1873 it has flowed in 1876, 1877, 1881, 

 and 1882, and it is now flowing in 1883. On some occasions it 

 commences flowing in summer; this year the flow of the stream 

 will probably cease in June or July. 



A hearty vote of thanks was accorded to Mr. Evans at the close 

 of his address, on the proposition of the President of the Geologists' 

 Association, Dr. Henry Hicks, seconded by Mr. Littleboy on behalf 

 of the Hertfordshire Natural History Society. 



The course of the Bourne was then followed to Bourne End, 

 where the stream flows into the Bulboume. In a portion of its 



