CLYDE SEA AREA. 



53 



The averages taken from Table XV., omitting the cases of no change of temperature, 

 are shown in Table XVI. Koughly speaking, the fall of temperature is one-third more 

 rapid than the rise. 



Table XVI. — Average Daily Rate of Heating and Cooling in F.° 





Whole Mass. 



Surface Layer. 



Bottom Layer. 



Heating. 



Cooling. 



Heating. 



Cooling. 



Heating. 



Cooling. 



Average rate, 

 Maximum rate, . 

 Number of cases, 



+ 0-057 



+ 0-110 



19 



- 0075 

 -0-180 



22 



+ 0-096 



+ 0-324 



20 



-0-120 



-0-375 



24 



+ 0-069 

 + 0-219 



20 



-0-086 

 -0-250 



22 



These averages are, of course, not strictly comparable, as they are not the averages of 

 equal periods ; but they are quite trustworthy for the comparison of temperature changes 

 at surface and bottom with those of the whole mass. The surface layer heated and cooled 

 one-third more rapidly than the bottom layer, while, on account of intermediate changes, 

 the whole mass of water changed its temperature more slowly than either of its extreme 

 surfaces. The much more rapid progress of cooling than of heating, indicated by the 

 greater positive than negative slope of the vertical curves, and demonstrated by the 

 averages given in Table XVI., is probably entirely due to the aid which downward 

 convection gives to the processes of heat transference by conduction, and mixture by 

 winds or tidal currents. 



Homothermic conditions preponderated in the months of falling and minimum 

 temperature, but were restricted to the lower layers, and sometimes lost altogether during 

 the months of rapid heating and maximum temperature. Observations of density at 

 Garroch Head were not made as often as was to be wished. The average of six at wide 

 intervals gave the surface density at 60° as 1*02417 and the bottom 1 '02504. This gives 

 for homothermic conditions a range of —0*00087, which for the greatest positive slope 

 ( + 9°*7) would be increased to -0*00226, and for the greatest negative slope (-3°*2) 

 diminished to —0*00050, which might lead to a reversal of the density gradient very 

 readily. The downward convection, which occurs at intervals during the months of 

 cooling, naturally tends to perpetuate a homothermic state, and so make it characteristic 

 of the season ; but even during the period of warming, the heterothermic condition, 

 incidental to the gain of heat by surface exchange, may be overcome in a day or two by 

 the influence of strong wind from a direction which ensures complete mixing of the water. 



Some sets of consecutive curves showing the phenomena of temperature changes may 

 be cited as characteristic. 



Fig. 13, Plate XXIV., shows very well the transition from a marked positive to a slight 



