Mr. Hen wood on Wells in Cornwall. 419 



On comparing the measurements of October and November, 

 it appears that in that interval the wells in the slate were sub- 

 siding, whilst those in the granite were rising. 



On the subject of the proportion of rain which finds its way 

 to the rivers, Mr. Thomas has made some very pertinent re- 

 marks §. He says, " From surveys of the waters of the river 

 Fowey, it was calculated that the quantity of water in the 

 month of April 1825, averaged at the lowest, for every acre, 

 about 160 cubic feet per day; and more than double that 

 quantity when the river was swollen with heavy rains. In 

 June 1826, which was a dry time, the quantity was reduced 

 to about 75 cubic feet per day for each acre." 



Through the kindness of John Taylor, Esq. F.R.S., and of 

 Captain Absalom Francis, I am favoured with engine reports 

 from the mines of the register of a rain-gauge kept at Coeddu, 

 in Flintshire ; and as they obligingly permit me to use them, 

 I will endeavour to apply them in the same manner I have 

 followed for the mines here ||. 



As my information of the Flintshire mining-field is derived 

 only through correspondence, these investigations are not so 

 satisfactory to me as my former ones, in which all the materials 

 were under my own eyes. But as I believe they will in the 

 present state tell us more than we had before known, in the 

 absence of what we could wish, we must use what we can obtain. 

 Captain Francis informs me, that " after the mines have 

 reached a certain depth, the quantity of water depends much 

 more on horizontal, than on vertical extension. The increase 

 of water in the mines in the limestone is felt generally within 

 a i'ew hours after it begins to rain, notwithstanding the great 

 quantity which is intercepted by adits. It is by no means un- 

 usual by the explosion of a single charge of gunpowder, to 

 admit a stream of water sufficient to keep an 18 or 20-inch 

 pump at work. Some of the veins are worked from the lime- 

 stone into the slates; but this is not general. The cross 

 veins are often from 50 to 100 fathoms in breadth, and the 

 strata of limestone are much deranged by them, and by beds 

 of shale." 



* Wells 1 to 8, 12, 13 and 16, are within a circuit of two miles, and 

 all those in granite within three miles of one another : 1 and 2 are 

 within 100 yards of each other, as are also 6 and 7, and 10 and 11. For 

 the observations on 4, I am indebted to the kindness of Mr. Wm. Opie ; 

 for 5, to R.Thomas, Esq.; for 9 3nd 10, to R.W. Fox, Esq. ; for 14, to Mr. 

 Richard Grigg; for 1.5 and 18, to the Rev. H. T. Coulson; and for 17, to 

 L. H. Potts, Esq. M.D. ; all the others were made by myself. 



f From Mr. Giddy's Meteorological Observations in Phil. Mag. 



J These numbers represent the depth of the water. 



§ " History of Falmouth by R. Thomas," 8vo. Trathan, Falmouth, 

 page 50. 



|J Phil. Mag. and Annals, vol. ix. p. 1J0, and Lond. and Edin. Phil. 

 Mag. vol. i. p^287. 



3H2 



