CLYDE SEA AREA. 43 



mass of water, may thus be considered, if they be sufficiently numerous and the time suffi- 

 ciently short, as limiting strata of water ; and any marked rearrangement of the isotherms 

 must be looked upon as due to the effect of the mixture of contiguous strata, or the intrusion 

 or withdrawal of some of them. In a time-depth section the heating of water from 

 above by absorption of heat or affusion of warmer water on the surface is shown by the 

 isotherms striking deeper. Cooling from the surface by radiation or by the withdrawal 

 of warm upper layers, and the welling up of colder water from beneath, is indicated by a 

 rise of isotherms toward the surface. The spreading out of isotherms apart from each 

 other indicates a mixture of contiguous layers : the crowding together of isotherms may 

 be due to the introduction above or below of water, differing greatly in temperature, or 

 to a shearing motion squeezing the layers between successive isothermal sheets into 

 less space vertically. 



In the diagram of Plateau observations it will be seen that until 1 o'clock in the 

 morning all the isotherms had a downward trend, showing that heating was occurring 

 from above. Now, no heat was reaching the water from the sun, which set within half 

 an hour of commencing the observations. Nor was there a warm wind blowing, nor 

 was the temperature of the air much above that of the surface water. Moreover, 

 the maximum surface temperature occurred at midnight. It is perfectly obvious, then, 

 that the heating must be due to warm surface water flowing to the position of observa- 

 tion, and the ebb-tide carrying a strong current of warm surface water from the Arran 

 Basin explains the effect fully. This surface water was accumulating on the Plateau, 

 and displacing the homothermic layer of water derived from the Channel. The thin 

 slice of water colder than 48°"5 served as a complete proof of the truth of this theory. 

 At the time of observation (or, at least, a few hours previously), the mass of water 

 in the Channel south of Sanda showed a minimum temperature of 48 0, 2 at 20 fathoms, 

 and a temperature of 48°"7 on the bottom. Former experience indicates that the 

 water further west, toward the Mull of Cantyre, would be homothermic and warmer ; 

 probably — as indicated in the curve for mean temperature of the Channel — a little 

 over 49°. This warmer water would possibly pass through Sanda Sound, and so cover 

 the western part of the Plateau. But it is evident that a cold layer scarcely above 

 48° separated the warmer bottom water from the relatively hot surface layers, and 

 this is accounted for by the overflow from the West Arran Basin, which, although very 

 warm on the surface, was colder than 48° at the level of the edge of the Plateau. 

 The isotherm of 52° reached its greatest depth (5 fathoms) at 3 h 0, the lower isotherms 

 at 2 h 0, but the isotherm of 49° below the cold layer at l h 0. It was low-water at 

 3 o'clock, and then the sudden elevation of the cold layer from a mean depth of 1 5 fathoms 

 to a mean depth of 10 fathoms took place, and the upper isotherms commenced to 

 retreat toward the surface, while the mean temperature of the mass fell steadily, 

 though slightly. The great thickening of the lower homothermic stratum, and the 

 practically unaltered thickness of the cold layer above it, showed that the rising tide 

 was carrying in Channel water beneath, and apparently raising up and causing to flow 



