44 DR HUGH ROBERT MILL ON THE 



back the warm upper layers, the isotherms of which were greatly crowded just above 

 the cold layer. 



Additional observations now suggest themselves which might have been made to 

 throw light on many points still left doubtful, but enough has been made out, I think, 

 to show that the flood-tide across the Plateau tends to carry in a mass of Channel 

 water along the bottom, while the ebb-tide rather affects the surface. Probably there 

 is always a current both in flood and ebb sweeping across the Plateau from surface to 

 bottom, but during ebb the surface current seems to do most of the work, and in flood 

 the under current. It is significant of this that the lowest isotherm in the diagram 

 is the first to show an inflection due to the setting in of the flood stream, and the 

 isotherms near the surface are the last. 



On the eastern side of the Plateau, as might be expected from its greater distance 

 from the Atlantic, the range between surface and bottom temperature is greater than 

 on the western side, but on the eio'ht occasions when observations were made on the 

 same or on successive days, on both sides of the Plateau, the mean temperature of the 

 vertical soundings never differed more than half a degree. 



The Arran Basin. 



This is the largest of the divisions of the Clyde Sea Area, and presents a peculiar 

 importance in being the intermediary in all interchange of water between the ocean 

 and the landward divisions. Water from the ocean, thoroughly mixed through all its 

 depth in the Channel, and passed inward across the Great Plateau, finds in the Arran 

 Basin a great reservoir in which it is mixed with landward water from rivers, estuary, 

 and lochs, and from which it passes, carrying the influence of the open sea into the 

 remotest recesses. The large island of Arran serves to divide the basin into three 

 parts, the peculiarities of which are clearly marked. The West Arran Basin is practi- 

 cally Kilbrannan Sound, and in its configuration resembles a sea-loch open at both ends. 

 On the south the opening to the ocean across the Plateau resembles the opening of a 

 sea-loch of the type of Loch Strivan. But the north or upper end sinks into the much 

 deeper trough of the Central Arran Basin, so that the fjord-like character is confined 

 to the steeply sloping parallel sides, and the gradually diminishing width from south 

 to north. 



The East Arran Basin, on the contrary, is broad and open on the south, meeting 

 the full breadth of the Plateau between Pladda and Turnberry Point. The trough 

 of water deeper than 50 fathoms runs parallel to the east coast of Arran, and keeps 

 close to the island, while on the east, a wide shallow, an extension northward of the 

 Plateau, sweeps round the shore of the mainland. The relatively shallow portion may 

 be said to occupy nearly two-thirds of the area of the eastern branch. The short 

 north-eastern branch running between Bute and the Cumbraes contains a deep narrow 

 basin barred off from the main trough, but of special interest, because in it the most 



