AND THE HERTFOUDSniRK BOURNE. 75 



rainfall 5 per cent, above it, but the spring yielded 16 per cent, 

 less than its average had been up to that time. In the five years 

 188(5-91 the winter rainfall was 10 per cent, below the average, 

 and the yearly rainfall 8 per cent, below it, the spring yielding 

 23 per cent, less than its former average. This is not a great 

 falling- off in relation to tlie rainfall ; but when we come to tlie 

 latest period, the five years 1891-96 (ending 31st March), there 

 is a very dilferent state of things. The winter rainfall in this 

 period Avas 10 per cent, above the average, and the yearly rainfall 

 1 per cent, above it, but the yield of tlie spring was 35^ per 

 cent, less than the average as stated by Mr. Muir in 1867, and 

 maintained to the early part of the year 1884 owing to tlie 

 exceptionally heavy rainfall for the eight years preceding that 

 date. This is the wettest period of such a duration since rainfall 

 observations were commenced in Hertfordshire (at Nash Mills) 

 64 years ago, but the average flow of the Chadwell Spring was 

 only just maintained. 



It will be observed that the very great diminution in the flow of 

 this spring in the five years 1891-96 coincides with the permanent 

 failure of the spring at Woolmers, and took place during a period 

 of more than average rainfall. 



The Hertfordshire Bourne. 



Until a few months ago the Chadwell Spring would have been 

 considered to be as good an example as we could well have of 

 a permanent Chalk spring, that is, of a spring which had never 

 failed and under natural conditions was never likely to fail. 



Let us now consider the case of a temporary spring rising from 

 the Chalk when the plane of saturation, usually below the surface 

 of the ground, is raised above the surface, the water from such 

 a temporary spring flowing down the bottom of a valley which is 

 usually dry, and thus forming a watercourse called a " bourne." 

 The stream which sometimes flows into the Bulbourne at Bourne 

 End. about half-way between Berkhamsted and Boxmoor, is an 

 excellent example of such a bourne. 



When I came to reside in Hertfordshire in 1874 I was told that 

 the Bourne flowed, or was supposed to flow, once in every seven 

 3'ears. It had been flowing in the spring of the previous year, 

 and therefore, on this assumption, most probably flowed in 1866 

 and was likely to flow again about the year 1880. I remember 

 that my first impression on hearing this was that it took about 

 seven years for the rain to accumulate underground until the Chalk 

 could no longer hold any more ; that the stream which then flowed 

 formed a kind of syphon which emptied the underground reservoir 

 to a certain extent, and that it took about seven years' rainfall to 

 fill it again. Tliis notion was dispelled by the Bourne itself in 

 1876, for it appeared in that year, three years after its previous 

 appearance, and I learned the coiTect theory from a paper by 

 Sir John (then Mr.) Evans on " The Hertfordshire Bourne," which 

 appeared in an early part of our 'Transactions.' ('Trans. 



