TRANSACTIONS OF SECTION B. 



739 



■water clianges comparatively little "with tlie season, always diminishing slightly 

 from the sea towards the head of the various lochs. The surface water also 

 freshens close to the shore and towards the upper part of the lochs ; but its salinity 

 at any time depends largely on the actual rainfall and on the height and steepness 

 of the surrounding mountain walls, being sometimes quite fresh and freezing in 

 severe frosts. The saltest surface water was almost invariably found near the 

 Otter Spit in Loch Fyne fifty miles from the open sea ; the tidal current moving 

 from deep to shallow water carries up the Salter lower layers. Wind currents 

 produce even more striking effects. When a gale blows dovm a loch, the 

 saltest surface water is found at the head, even though a stream enters in the 

 immediate neighbourhood, in the position where normally it is ireshest. This is 

 in consequence of the upwelling of salt water from beneath to replace the surface 

 layers driven away by the wind, and fully confirms Mr. Murray's theory (suggested 

 by temperature observations) of the circulation of water in enclosed basins. 

 Following are the average results of from eight to fourteen observations at a 

 iew selected stations spread over two years, the density being tiaat at 60° F. 

 (15-56° C). 



5. Sea Temperatures on the Continental Shelf.^ 

 By Hugh Robeet Mill, B.Sc, F.B.S.E. 



The name ' continental shelf ' is applied to the shallow and gradually sloping 

 ground from the sea-margin out to the 100-fathom Hue, beyond which the descent 

 to abysmal depths is abrupt. The British Islands rest on one of the widest con- 

 tinental shelves in the world, and the present paper summarises observations made 

 by the author on its western edge. The observations were carried out at the 

 request of the Fishery Board for Scotland on board H.M.S. 'Jackal ' in July and 

 August 1887, and consisted of lines of serial temperature soundings from the 

 north-west coast of the island of Lewis seawards to beyond the 100-fathom line. 

 This portion of the continental shelf is terraced, and the slope varies in different 

 places. It is broken by the long Flannan bank and the small circular bank of 

 St. Kilda, and grooved by several deeper channels. The form of the curves of 

 vertical distribution of temperature and the direction of the isotherms in the 

 temperature sections show that the water reaching the seaward edge of the shelf 

 from the ocean consists normally of a layer more than twenty-five fathoms deep at 

 a uniform temperature of 56°, resting on a mass of water at a temperature of 48° 

 or 49°. The action of waves partially mixes the two layers, and they were found 

 separated by a zone about fifteen fathoms thick, in which the temperature changed 

 rapidly with depth. The prevailing westerly wind and eastward tidal current 

 produced changes in this typical arrangement of layers in exact relation with the 

 configuration of the sea-bed. The warm layer, meeting no resistance from in- 



' Published in extetiso in the Scottish Geographical Magazine, Oct. 1888. 



3 B 2 



