090 DR HUGH ROBERT MILL ON THE 



and one yearly minimum in February corresponding to the maximum rainfall of December 

 and January. It is quite to be expected that increased rainfall freshens the water more 

 rapidly than diminished rainfall increases its salinity, since evaporation is most active 

 (according to Mr Symons' recent results # ) in July, and is practically at a standstill from 

 October to February. Thus in the four rainiest months on the Clyde drainage area 

 there is almost no evaporation, while during the four driest months evaporation is active. 



Salinity and Rainfall of Seaward Portion of Area. — The mean rainfall curve of this 

 division (Plate XII.) is similar in form and period to that of the landward division. The 

 minimum monthly rainfall is 0*4 inches lower, and the maximum 1*8 inches lower than the 

 other. The actual rainfall curve runs closely parallel to that of the landward division, but 

 its maxima occurred in September and December. The surface and bottom salinity curves, 

 as might be expected, run much more nearly parallel to each other than do those of the 

 landward division. The surface landward curve stands by itself in its great range and 

 irregular variations, but the bottom landward curve accords very well with the seaward 

 ones, being intermediate between the seaward surface and bottom. The seaward curves 

 show a steady rise to a maximum in August, a steady fall to November, a sudden rise in 

 December, a nearly equal fall in February, and then a steady rise to July or August. 

 The double minimum in November and February, with the intermediate maximum in 

 December, appear to correspond to the double maximum of rainfall in September and 

 November and the intermediate minimum in October. If this be so, the change of 

 salinity affects the whole mass of water, from surface to bottom, within two months of 

 the rainfall producing it. There is no evidence to show that the sea water entering the 

 Area across the Great Plateau varies sufficiently in salinity from month to month to 

 produce any marked effect. Yet, without invoking an explanation from the open sea, it 

 is difficult to understand how the comparatively slight reduction of rainfall, in a season 

 when the evaporation must be very small, can effect an increase of salinity throughout 

 the whole mass of the water. 



The main result from the consideration of these curves may be stated in the words — 

 Salinity throughout the whole mass of water varies in some inverse ratio to the rainfall 

 of the previous month. 



Quantitative Relation between Rainfall and Salinity. — The period April to September 

 falls within the observations of the two successive years 1886 and 1887. In order to 

 endeavour to connect rainfall and salinity in a quantitative manner, the rainfalls of each 

 of these and of the three previous months were written out, and the salinities of the 

 middle of the corresponding months taken from the mean curves of landward and sea- 

 ward divisions, surface and bottom, were also tabulated. The differences between the 

 two years were calculated, as given in Table XXII., and these differences were plotted in 

 curves (not reproduced in this memoir) in order to bring out any relation which might 

 exist. On account of the disturbing influence of wind and local freshening on surface 

 salinity, a close coincidence is a priori less probable for the surface curves than for the 

 bottom curves. 



* British Rainfall, 1889. 



