CLYDE SEA AREA. 139 



XII. 28th September 1887. — In this section autumnal conditions are well shown. 

 The warmest water, homothermic at 5 4° '5, was in the Arran and Dunoon Basins, but 

 gradually gave place to heterothermic conditions on the Bute Plateau, where lower tem- 

 peratures reigned on the bottom, and inside the loch basin lower still. In the loch the 

 surface water had cooled down a little, so that there was an intermediate layer at a 

 slightly higher temperature, but a slight tilt of the isotherms caused the upper cold layer 

 to thin away toward the head of the loch where the warm zone came to the 

 surface. A slight north-easterly breeze was blowing down-loch at the time. Below the 

 level of the bar the isotherms were apparently horizontal. 



XIII. 2nd December 1887. — Four of the most interesting sections made during the 

 whole course of the work on the Clyde Sea Area record the results obtained by Dr Murray 

 in December 1887, and three of these he has described* in an article discussing the 

 effect of wind on the circulation of water. They are included here in order to complete 

 the set. On December 2nd the ordinary winter condition was established, a mass of 

 warm water occupying Loch Strivan covered over and shut in seaward by colder water. 

 The isotherms dipped strongly seaward, showing an up-draught of the warmer water at 

 the head of the loch, and an in-draught of the colder bottom water from outside across 

 the Bute Plateau. The wind was blowing a stiff breeze from W. by N., mainly trans- 

 verse to the loch, but with a down -loch component which would become more powerfully 

 felt at the junction with the Dunoon Basin, off Bogany. 



XIV. \Ath December 1887. — The change brought about in the twelve days elapsing 

 since the last section was drawn was very remarkable. At the earlier date the surface- 

 temperature was everywhere 48° or more, on the 14th it was 45° at the mouth of the 

 loch, and diminished to 39°*3 at the head, where the water was muddy and quite fresh. 

 The most interesting feature was a Sprung schicht in which the temperature rose from 

 44° to 48° in a fathom and a half, at a mean depth of 6 fathoms. Beneath, the tempera- 

 ture was almost homothermic at 49° ; above, almost homothermic at 43°. A gale 

 from the south-west was blowing up the loch, banking up the cold fresh- water at the 

 head, and causing the Sprung schicht, raising the warmer deep water to the surface at 

 the mouth, where the isotherms of the Sprung schicht spread out like a fan. The dis- 

 turbance due to wind at no point reached deeper than 10 fathoms. 



XV. lbth December 1887. — This section represents the distribution of temperature 

 twenty hours later than No. XIV. ; in the meantime, the wind had fallen calm, and changed 

 to a northerly direction. The air was cold, and ice formed on the nearly fresh water at 

 the head of the loch. The up-loch dip of the isotherms had almost disappeared, except 

 in the upper layer. The Sprungschicht had risen to 5 fathoms at the head, and as its 

 component isotherms spread out down the loch, they assumed a marked seaward dip, 

 indicating a reversal of the direction of vertical circulation. The homothermic layer in the 

 deep part of the loch remained as before, but on the Bute Plateau the conditions had 

 become heterothermic throughout. 



* Scottish Geographical Magazine, iv. (1888), p. 351. 



