JOHN EVAlSrS THE HERTFOEDSHIKE BOITRNE. 139 



an inclination of 10 feet to the mile, it would be less than that at 

 which the water finds its way underground, and there would be a 

 constant stream of water thi-ough it, as it would afford the readiest 

 means of delivery from the underground reservoir. But take a 

 natural valley, with an inclination of 16 or 17 feet to the mile. 

 Under ordinary circumstances water finding its way at an inclina- 

 tion of 12 feet 6 inches to the mile would not show upon the 

 surface. Suppose, however, that in a veiy wet winter the water in 

 the subterranean reservoir was raised so as to be at an iaclination 

 of 25 feet to the mile, it would at once appear on the surface, 

 at the bottom of such a valley, because it would find its way more 

 readily over the surface than underground. 



Thus with regard to the valley of the Bourne. In certain years 

 the subterranean water is raised in the body of the hills, and this 

 valley, cut at right angles or nearly so to the main stream, inter- 

 sects the general surface of the plane of saturation, and the water 

 aj/pears on the surface. It, of coiu'se, is not the case that the 

 slope of any valley is absolutely ia one uniform line ; generally 

 speaking there are some small undulations. In looking at the 

 Bourne, I found this theory of intermittent streams being due 

 to the intersection of the plane of saturation very prettily illus- 

 trated, because in one place I found the Bourne running, and when 

 I arrived at a certain spot it had disappeared ; further on it ran 

 again, and again disappeared. The plane of saturation was more 

 even than the surface, and where there was an elevation of the 

 surface the water found its way underground instead of running 

 over it. At one spot, where there was a depression, there was 

 water five or six inches deep, all over the road, and I found that 

 while the water was visible near Harratt's End Lane, it disappeared 

 further down the valley. Past Bottom Farm the water ran in 

 considerable volume, but before it arrived about haK-way to 

 Bourne End, it disappeared altogether, and though a pond which 

 was diy last year had six feet of water in it, the stream did not 

 run out of it, but re-commenced running a few yards below it. 



I must now refer to the obseiwations made as to the rainfall 

 during the years which have been marked by the fiowing of the 

 Bourne. I mentioned that in March, 1853, it was fiowing. I 

 find that dui-ing the six winter months of 1852 and 1853 there had 

 been through our gauges for ascertaining the amount of percolation, 

 10'74 ins. of rain. The stream was running in March, and in 

 March, 1853, the percolation through the soil had ceased. But 

 during that winter a much larger quantity of water than usual 

 found its way into the Chalk, for the average winter percolation 

 is not more than five or six inches. Taking another of these 

 years; the Bourne was flowing on February 16th, 1873. The 

 amount of rain which during the six winter months found its way 

 to the subterranean reservoir was 11-25 inches. This year the 

 Bourne did not commence flowing until April, and judging from 

 our gauges, which at any rate give an approximate measure, 

 its appearance is due to a less amount of infiltration of water than 



