394 PHYSICAL CONDITIONS OF 



of tidal mixture almost homothermically, until at the period of annual 

 maximum the surface reaches about 56 in August, and the bottom about 51* 

 in October. When cooling commences it takes place almost simultaneously 

 throughout the whole depth, so that although nearly nine months are 

 required to raise the temperature at the bottom from the minimum to the 

 maximum, only three months are necessary for reducing it from the maximum 

 to the minimum. During the later months of cooling the surface water 

 remains colder than that beneath (see curves in Fig. 3). 



From the Arran Basin communication is had directly with Loch Fyne and 

 the Dunoon Basin, the latter leading to Loch Goil. 



Loch Goil, much more isolated than the Arran Basin from oceanic 

 influence, shows all the characteristics which distinguish the Arran Basin 

 from the Channel, but in an exaggerated degree. At the annual minimum 

 in early spring the mass of water is homothermic, and practically at the 

 same temperature as in all other parts of the Area, but the surface alone 

 heats up rapidly as the season advances. The mass of water below the level 

 of the bar heats very slowly, and only to a slight extent. It is probable 

 that heat from the surface-layers is transmitted downwards mainly by the 

 slow process of conduction, the maximum temperature at the bottom being 

 reached late in winter nearly six months after the date of the surface 

 maximum, and only a month or two before the minimum temperature 

 reasserts itself from top to bottom. The vertical distribution of temperature 

 is thus subject to remarkable changes. The curve of the minimum is homo- 

 thermic as elsewhere, the curve of heating shows a great range of temperature, 

 and when cooling sets in its upper part is reversed in direction. The cold of 

 winter may have reduced the upper layers of water to 34 or less, while the 

 heat of the previous summer is still being conducted downwards, and 

 the water remains warmest in the centre and colder both above and below. 



The conditions of Loch Fyne have been more fully investigated than 

 those of the other lochs. The shallow Gortans Basin, with its entrance and 

 exit restricted by bars, serves as an intermediate chamber between the 

 Arran Basin and the deep Upper Basin of Loch Fyne. The steep rise of the 

 sea-bed off Otter causes the flood tide to produce an updraughtof deep water 

 from the Arran Basin, breaking the warm skin of surface water in summer 

 and the cold skin of surface water in winter, and filling the Gortans Basin 

 with nearly homothermic water of relatively high salinity. This water is 

 passed on to the Upper Basin at Minard, and occupies the gently sloping bed 

 of the loch as far as Furnace ; but there, where the Upper Basin drops 

 abruptly, the incoming tidal water seems to intrude itself like a wedge 

 horizontally into the mass of the water of the Upper Basin, not directly 

 affecting the deeper layers, which thus remain nearly as isolated as those of 

 Loch Goil, and present a similar thermal regime (see curves in Fig. 4). 



In the deep lochs it will be noticed that the annual range of temperature 

 becomes less as the depth is greater, and it would seem from a consideration 

 of the temperature curves that if the depth of Upper Loch Fyne were 

 increased to 150 fathoms, the temperature of the deepest water would retain 

 the minimum temperature practically unchanged all the year round. 



The three series of curves in Figs. 2-4 show the temperature at various 

 depths in three typical regions as observed in 1886-87 during a complete 

 cycle of heating and cooling. The numbers attached to the curves indicate 

 their position in this cycle as follow : (1) Minimum, (2) Early heating, 

 (3) Later heating, (4) Maximum of upper layers, (5) Maximum of mean 

 temperature, (6) Early general cooling, and (7) Later cooling. 



In April, 1886, the water of the whole area from a depth of 5 fathoms to the 

 bottom had a temperature so nearly uniform that it did not vary more than 

 O2 from 42'0 F. ; at the corresponding date in 1887 it was within 1 of 

 44 F. : in 1888 within 1 of 43 F. : and from numerous though less 



