658 DR HUGH ROBERT MILL ON THE 



various parts of the drainage area, and thus they serve only as an indication, not a measure 

 of the seasonal range and fluctuations of precipitation over the district. It is satisfactory 

 to note that although the May miDima, both in 1886 and 1887, are pushed forward to 

 June in each year, and the maximum occurs in November 1886 instead of the following 

 January, yet the maxima and minima are distinctly marked, and may be reasonably 

 expected to give rise to something closely resembling the normal changes in the salinity 

 of the water. (See curves in Plates XI. and XII.) 



The question of the amount of river-discharge corresponding to a given rainfall is 

 complicated and difficult. Considering a very long period and a very large surface, the 

 rain which falls on a given drainage basin is almost entirely accounted for by the quantity 

 which is discharged into the sea by rivers, plus that which is returned to the atmosphere 

 by evaporation from the rivers and the wet earth, and by the transpiration of plants. 



In a small area considered for a limited time, other factors must receive attention. 

 A certain proportion of the precipitated moisture is evaporated directly, a certain 

 proportion runs off the land surface and enters the sea by streams and rivers ; a certain 

 proportion also is taken up by plants and animals, and is not fully returned until 

 their death. This acts in spring by reducing the amount of surface-drainage water, as 

 the growing vegetation requires an increasing amount of liquid in its tissues ; in autumn 

 the gradual ripening and withering liberates this moisture, most of which is taken up by 

 the atmosphere, although some, as in the case of heavy dews, may return to the surface- 

 drainage. Again, rain falling after a prolonged drought is not so effective in swelling 

 rivers as rain falling after a heavy rainfall. In the former case much is retained by 

 capillary attraction in the soil and amongst the roots and fibres of grass and mosses, 

 whence it drains out slowly, and ultimately reaches the streams or gradually 

 evaporates. A still more serious objection is presented by the underground circulation 

 of water. Water-bearing strata, after a prolonged drought, not only absorb large 

 quantities of the rainfall percolating through the surface soil, but it is quite possible to 

 conceive cases where heavy rainfall may thus be conducted underground across a water- 

 shed and reappear in springs on an area where the rainfall has been much less, and from 

 which the drainage flows to a distant sea. It appeared unnecessary to attempt any 

 detailed study of the geology of the district in order to arrive at an estimate as to the 

 amount of water falling on the Clyde Sea Area drainage basin that might thus cross 

 the watershed, nor was it possible to estimate the influence of the other causes except in 

 very rough manner. With the advice of Dr A. Buchan, and in the light of several sets 

 of published experiments on evaporation, I assumed that the average evaporation over the 

 land surfaces of the West of Scotland amounted to 15 inches in the year, including for 

 this purpose all the narrow arms of the sea as land, while for the wide water surfaces of 

 the Arran Basin and Plateau the annual evaporation was taken as 25 inches. Mr G. J. 

 Symons was subsequently kind enough to draw my attention to a prolonged and very 

 valuable series of experiments reported in his British Rainfall for 1889. Here he 

 shows that in London during the five years 1885-89 the amount of annual evaporation 



