CLYDE SEA AREA. 693 



surface and bottom steadily increases. The actual figures are given in Table XXV. The 

 great range and variability of the difference between surface and bottom salinity is due 

 almost entirely to the great, sudden, and erratic changes of surface salinity as affected by 

 temporary conditions of rainfall, frost, and wind. It is apparent that the range as well 

 as the amount of difference between surface and bottom water is least toward the open 

 sea, and greatest in the landward basins. Samples of water from intermediate depths 

 were frequently taken, and Table XXVI. contains a statement of all such cases. Here 

 there is an opportunity of breaking up the change of salinity into two parts, namely, from 

 the surface to the intermediate depths, and from the latter to the bottom. The summary, 

 including the results of eleven stations, for which there were three or more observations 

 of intermediate salinity, shows that on the average, when the intermediate sample was 

 taken so as to make its distance from the bottom 3 '8 times greater than from the surface 

 (or roughly at one-fifth of the depth), the change of salinity in the upper division was 

 five times as great as in the lower. In other words, five-sixths of the total difference of 

 salinity between surface and bottom water occurs in the superficial layer one-fifth of the 

 total depth. Hence it appears that in a typical case there is a sheet of comparatively 

 fresh water, the salinity of which increases rapidly with its depth, resting upon a mass of 

 much Salter water four times as deep, throughout which there is a very slight increase of 

 salinity with depth. Supposing that the rate of increase of density with depth through- 

 out each layer is uniform, the average salinity would be given by the formula 



^ + 45±5=5*or^±«^*=a (1) . 



when A = surface salinity, B = bottom salinity, n = salinity at one-fifth of depth, and x = 

 mean salinity. But 



w-A = 5(B-w)or5B+A = 6«. .-. n = ^tt^ .... (2). 



Substituting in (1) we get , 17A + 0'82B = a;, and this formula will in an average case 

 give the mean salinity at any position where the bottom and surface salinities are known. 

 Of course the assumption of a uniform rate of change of salinity from surface to bottom 

 of the two sections is not correct ; but there are no data to determine the exact curve of 

 salinity, which must vary greatly with circumstances, and the assumed uniform rate gives 

 a first approximation to the truth, all that can be looked for in the circumstances. 



The summary gives, in column A, the numerical ratio which the depth of the layer 

 below the intermediate point bore to that above it ; and in column B the ratio of the 

 change of salinity above the intermediate point to that below it. 



It will be noticed in the summary that at the head of Loch Goil and of Loch 

 Strivan the change of density in the superficial layer is less than that in the lower ; this 

 is probably a consequence of the very pronounced action of wind at the head of these lochs 

 on the days when observations were made. 



In order to estimate the average salinity in a vertical section at each station, the 



