Ixvi PROCEEDINGS, 



of the ground had another effect above the lane, for here the water 

 had accumulated, forming a large but shallow pond, above which 

 it was still flowing freely for nearly half a mile, the source of the 

 Bourne at this time being just where the valley takes a sharp turn 

 to the north-west. It was here flowing sluggishly through marshy 

 ground, its first appearance as a running stream being at the 

 extreme comer of a field of grass, the next, a ploughed field, 

 merely having indications of surface -saturation in the presence of 

 mud. The valley which the Bourne has formed continues for 

 a mile and a half beyond this point, nearly to Cross Oak, the bed of 

 the stream from the head of the valley to about half a mile below 

 Bottom Farm marking the boundary between the counties of Herts 

 and Bucks. 



The object of the expedition, — to trace the Bourne to its source, — 

 having been achieved by the President and the majority of the 

 party, the valley was descended to Bourne End without any mis- 

 haps in crossing the stream, and tea was found to be waiting, not 

 the first edition, for some of the members had returned earlier and 

 had tea with others who never reached the main party. 



Before the party dispersed, the President made some remarks on 

 the phenomena presented by the Bourne. He said that the general 

 slope of the country here was from north to south, and that the 

 inclination of the underground water-level in the Chalk roughly 

 followed the slope of the surface. Rain would percolate into the 

 Chalk until it was saturated, and when any permeable formation 

 was full of water it was practically impermeable, for it could not 

 receive a further quantity. With an average rainfall the water 

 which percolates into the Chalk remained underground, but with 

 a certain increase the slope of the underground water-level became 

 greater than that of the surface of the ground, and the water 

 consequently rose above the surface, the underground reservoir 

 overflowing, and in this instance forming the Hertfordshire Bourne, 

 There were other bournes in the southern counties, such as the 

 Croydon Bourne, which commenced to flow in February, nearly two 

 months earlier than this, the reason probably being that the out- 

 crop of the Chalk at Croydon was narrower and steeper. 



Mr. Hopkinson then gave a list of the recorded instances of the 

 flowing of the Hertfordshire Bourne,— in 1853, 1873, 1876, 1877, 

 1879, 1881, and 1883, — with the dates of the commencement of the 

 flow and the position of the source ; and also the average rainfall 

 in the neighbourhood for the last four months of the year 1896. 

 The mean of ten stations, — two at Tring, two at Cowroast, one at 

 Northchurch, two at Bcrkhamsted, one at Great Gaddesden, and two 

 at Hemel Hempstead, — was as follows : — September, 6'80 inches ; 

 October, 3*57; November, 1*26; December, 4*25; giving a total 

 for the four months of 15-88 inches, which is nearly six inches in 

 excess of the average. The rainfall had also, he said, been rather 

 heavy during the first three months of 1897, especially in March, 

 but the present month of April, in which the Bourne commenced to 

 flow, was so far drier than usual. 



