AND THE nEBTFORDSHIRE BOURNE. 77 



has ceased, this interval beinj^ occupied by the water which falls 

 upon the surface of the ground percolating through the Chalk to 

 the normal ]>lane of its saturation, whifli has to be raised to 

 a greater angle with the liorizontal than that of the bottom of the 

 valley, and at the point where the plane of saturation intersects 

 the bottom of the valley, the water commences to flow above the 

 surface of the ground, there being at this point a temporary spring. 

 The inclination of the valley of the Bourne is from about 15 feet 

 to the mile in its lower portion to 20 feet near its head. The 

 fi-ictional resistance of the pores of the Chalk against the force of 

 gravitation causes water in it to flow from the higher ground to the 

 rivers at an inclination of at least 12 feet to the mile. In wet 

 weather this inclination is much increased, and it is evident that 

 the stream will only flow when it is greater than the inclination of 

 the valley of the Bourne. 



It is not, however, my intention to go fully into the theory of 

 bournes, for that has already been done by Sir John Evans in the 

 paper to which I have referred. I merely wish here to attempt to 

 show what amount of rain is required to cause our Bourne to flow, 

 by comparing the recorded instances of its flow with the rainfall 

 of the preceding winter or year, the winter and the year being the 

 six months or the twelve months ending 31st March. The Bourne 

 has certainly flowed ten times during the last half-century, and 

 in twelve years during this period the mean rainfall in Hertford- 

 shire has exceeded 30 inches. The Bourne has never flowed when 

 less than this amount of rain has fallen, and therefore only twice 

 has it failed to flow when the year's rainfall has exceeded 30 inches. 

 In fact we can only be certain that this has occurred once, and that 

 was in the year 1892, when the year's rainfall was 30-21 inches 

 (to 31st March), and the winter's 16-61 inches. The other year 

 with no record of its flow and no grounds for inferring that it must 

 have flowed, was 1861, when the year's rainfall (to the usual date) 

 was 31-31 inches, and the winter's 11-81 inches, this probably 

 being too small a winter's fall to cause it to flow. That it did not 

 flow in 1892 may be owing to the winter of 1890-91 having been 

 a dry one. 



The following table (Table I, p. 79) gives the mean summer, 

 winter, and yearly rainfall in Hertfordshire (the mean of ten gauges) 

 whenever the Bourne has certainly flowed, with the ratio to the 

 average for the half-century 1842-92. It will be seen that the 

 Bourne has never flowed, so far as we know, unless there has been 

 an excess of at least 4-^ inches of rain in the year, all due to the 

 winter's fall (1896-97) ; of at least 5^ inches, of which the winter's 

 excess was 2 inches (1859-60) ; or of at least 6^ inches, of which 

 the winter's excess was H inch (1878-79). The only other years 

 with a rainfall above 30 inches are 1860-61, when the excess was 

 5 inches and the winter's defect 1 i inch, and 1891-92, when the excess 

 was 4 inches, the winter's excess being 3^ inches. On the first of 

 these occasions the Bourne may have flowed, but, owing to the small 

 rainfall in the winter, it is scarcely likely to have done so ; on the 



