THE FARMER'S MAGAZINE, 



239 



water to fall by its own gravity. There are various 

 curious matters about the flow of these intermittent 

 springs, which puzzle even the philosopher to ex- 

 plain. For instance, Mr. J, B. Knight, of West 

 Lodge, in Dorsetshire, tells us, after graphically 

 describing these waters or winterbournes, w hich 

 annually burst at the heads of valleys enclosed by 

 chalk hills, in Dorset and Wiltshire, and commonly 

 afford the prelix of Winterbourne to the parishes 

 where they occur, such as Winterbourne Tarrant, 

 &c., that one curious fact in relation to the annual 

 bursting of these springs is, that their " breaking," 

 as it is termed, is always accompanied with strong 

 gales of wind, generally from the south-west, with 

 rain; h\xt without a strong gale they never break, 

 however wet the season. The peasantry of Dorset 

 tell you that the first outbreak of these bournes 

 was never yet beheld by any mortal eye ; that not 

 only do they comelike the witches in gales of wind, 

 but that, like the weird sisters, they are at first 

 invisible. 



The origin of these annual bournes is pretty evi- 

 dent ; all the rainfall upon the chalk formation 

 either evaporates or sinks into the earth ; there is 

 little or no surface drainage there. In the summer 

 months the rain which it receives is almost entirely 

 evaporated; there is little or no overplus to feed its 

 bournes. But in the autumn and winter months 

 the case is reversed ; the smaller portion of the rain- 

 fall evaporates from the surface of the soil ; the 

 larger portion descends beneath it, and either en- 

 larges the flow of the constant streams of the chalk, 

 or forms new outbursts or winterbournes. 



If, however, the rainfall considerably exceeds the 

 average annual amount, v/hy then it happens that 

 in certain places the old streams or bournes, are 

 either unable to discharge the unusual amount of 

 water with which the chalk hills are surcharged, or, 

 what is nearly the same thing, there are certain 

 veins of earth in the chalk which filter the water 

 more slowly than the chalk which rests above them : 

 in either case the result is the same — the water is 

 retarded in its descent, and is diverted into other 

 outlets on a higher level than those of the old 

 springs, and these irregular outpourings are the 

 bournes of Croydon and other places. 



Let us for the better understanding of this follow 

 the rainfall upon the chalk a little more closely. The 

 annual average rainfall in Surrey is about 24 inches, 

 or 2,400 tons per acre ; of this quantity we learn 

 from some experiments carried on for some years 

 by Mr. Dickenson, on the chalk soils of Hertford- 

 shire, the evaporation by the sun and winds carries 

 off about 14 inches, or 1,400 tons, and about 10 

 inches, or about 1,000 tons per acre, sink into the 

 earth, and are discharged by the springs or other 

 natural drains. 



When, however, an unusual fall of rain occurs, 

 say thirty inches of water during a similar period, 

 a very different state of affairs take place. 3,000 

 tons of water per acre now descend upon the land. 

 The evaporation from that soil, however, remains 

 nearly the same,"viz., about H^inches in depth, or 

 1,400 tons of water per acre, whilst the drainage 

 water is increased from 10 to 16 inches, or from 

 1,000 to 1,600 tons per acre; an outlet, therefore, 

 is now needed that will convey 600 toRa of water 



per acre more than in years of average rainfall. If 

 this tracing of the course of the drainage waters of 

 the chalk is correct, then it ought to follow that 

 the Bourne at Croydon, and other places of its 

 irregular outpouring, should make its appearance 

 soon after any unusual annual rainfall. Now, 

 I nd that whenever the rainfall in one year is 

 about 30 inches the Bourne makes its appearance 

 in the close of the year, or early in the ensuing 

 spring; and that whenever the rainfall considerably 

 exceeds 30 inches in the year, as in 1818 (33"4 

 inches), 1821 (34-5 inches), 1824 (30-3 inches), 

 1841 (33-3 inches), 1852 (342 inches), the Bourne 

 flows copiously. 



When the spring makes its appearance by the 

 hill-side, it begins to travel by its own gravity to 

 mingle with other springs. As soon, however, as 

 the water bursts forth, evaporation from it begins, 

 and if its course is considerable before it meets 



