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Part III. — Seventh Annual Report 



waters. What I do consider important is the fact of the difference in the 

 chemical composition of these waters and the application of the method I 

 have adopted for the recognition of such differences. 



To return to the Moray Firth, a further confirmation of the view that 

 the waters which filled it in August and September 1883 really came 

 from the surface of the Atlantic is to be found in the relatively high 

 temperatures observed by me on that occasion along the bottom as well as 

 at the surface. The lowest bottom temperatures which I met with were 

 51°-8 and 51 0, 9 F. respectively, while the mean bottom temperature cal- 

 culated from the observations made in the Moray Firth at depths not less 

 than 8 fathoms and excluding those observed in the shallow waters of the 

 Cromarty and Inverness Firths, and those near the mouth of Spey, was 

 52 0, 5. Now Dr Mill in August 1886 found that in the Moray Firth the 

 great mass of the water, all that lying below a depth of 6 fathoms off the 

 Caithness coast and 12 off the Morayshire coast, was at a temperature 

 between 52° and 50° "5 ; on one occasion he found the temperature near the 

 bottom at a depth of 33 fathoms as low as 49° *5 — the mean of 10 bottom 

 temperatures in the Moray Firth being only 51 '0. 



The question now arises as to how the interchange of waters in the 

 Moray Firth comes about. Dr John Murray's extremely suggestive 

 observations with regard to the action of wind on the surface of the water 

 in the Scottish lochs appear to give the clue. Suppose at a time when 

 the Moray Firth is filled as it was in August 1883 with warm surface 

 Atlantic water, that the wind should commence blowing steadily from 

 the south-west. The surface water in the wedge-shaped estuary would 

 then be blown out to sea, wliile the wind acting on the entire surface of 

 the water in the estuary would prevent more or less completely the setting 

 in of any surface return current. It follows that the surface water blown 

 out to sea must be replaced by water flowing in along the bottom. An 

 undercurrent would set in therefore flowing from out to sea, towards the 

 head of the Firth, that is, in an opposite direction to that of the wind. 



Now in the northern area of the North Sea there is a lower layer of 

 water markedly colder than the water which flows in from the Atlantic 

 round the North of Scotland and through the English Channel, and 

 further the analytical results do bailed ab or o clearly show that these 

 waters differ not merely in temperature but in chemical composition. 

 If then the wind continued blowing steadily for a sufficient time in the 

 same direction, the result would be that the layer of water which at the 

 commencement was at the surface in the Moray Firth would all be blown 

 out to sea, and that the water which was originally at the bottom of the 

 Firth would come to the surface, and its place be taken in turn by the 

 colder water from the North Sea. We should thus expect to find at this 

 stage the surface water in the Moray Firth different in chemical composi- 

 tion from that at the bottom. If, however, the wind continued blowing 

 in the same direction for a sufficiently long period, the whole of the water 

 originally present in the Firth would be blown out to sea and replaced by 

 water flowing in along the bottom, and thus the action of the wind would 

 result in an entire and complete replacement of the warm Atlantic water 

 originally filling the Moray Firth by bottom water from the North Sea. 



This process may no doubt be followed to a certain extent by 

 means of the thermometer and hydrometer without the help of any 

 chemical analysis, but the indications thus arrived at are at once less 

 easily interpreted and more uncertain than those given by the method 

 described above. Moreover, the use of purely physical means could not 

 lead to the recognition of the very important distinction between the two 

 sets of waters, that, namely, of chemical composition. 



