78 Proceedings of the Royal Irish Academy. 



Derg, the average would doubtless be higher. We may take it roughly 

 as 36 inches for the entire basin. 



Prestwich calculated, from the observations of Harrison for eleven 

 years, and from those of Beardmore for eighteen years, that the 

 Thames discharge at Kingston averages 1,250,000,000 gallons daily, 

 from 3670 square miles, which is equivalent to 8 inches per annum. ^ 

 The average annual rainfall is 27 inches, so that the discharge is 

 somewhat more than one-third. The Severn discharges 1,600,000,000 

 gallons for 3890 square miles above Gloucester, equivalent to 10 inches 

 of rainfall. The average record here is 40 inches, so that the discharge 

 in this case is about one-fourth. For the purpose of comparing these 

 two areas and their discharges with those of the Shannon, it may 

 further be stated that about two-thirds of the Thames basin is occupied 

 by permeable strata; and it is to be expected that the proportion of 

 water evaporated therefrom would in the aggregate be less propor- 

 tionately than from the surface of the Severn basin, formed for the 

 most part of impermeable strata. The proportion of the rainfall 

 evaporated in the latter case is greater than in the former, and the 

 discharge consequently less in proportion to the rainfall. In the case 

 of the Shannon the evaporation must be very great, because of the 

 numerous lakes, marshes, peat-bogs, and protracted water-flow, in 

 streams and tributaries, within the low-lying, comparatively flat 

 basin.^ We do not, therefore, greatly err in comparing the circum- 

 stances determining the proportionate discharge of the Shannon with 

 those of the Severn rather than those of the Thames, and in estimating 

 the Shannon discharge as about one-fourth of the rainfall, that is to 

 say, 9 inches per annum. 



Analyses of the Shannon water, as carried out by Sir Charles 



1 " Anniversary Address," Quart. Journ. Geol. Soc, vol. xxviii., 1872. 



2 Mr. R. H. Scott, f.r.s., thought that the evaporation from a free-water surface 

 about equals the rainfall. Mr. C. Greaves, c.e., found that on an average of 

 fourteen years — 1860-1873— the rainfall of London was 25-721 inches and the 

 evaporation 20*613, and that in three distinct years the evaporation exceeded the 

 rainfall ; and the late Dr. Haughton, r.R.s., ascertained that on the average of two 

 years in Dublin, the evaporation fell short of the rainfall by only TOS inch. — 

 "Elementary Meteorology," by R. H. Scott, f.r.s., 6th edition, p. 102. The 

 Rothamsted averages for seven years — 1870-1877 — were 30*26 inches of rainfall ; 

 and 



Evaporated from or retained by soil (a clay-loam), 20 inches deep, 17*97 inches. 



40 „ „ 17*47 inches. 

 60 „ „ 17-40 inches. 



