Loch Lomond during the Autumn of 1885 315 



hence the temperature of the surface, and of the layers near 

 it, rises much more rapidly than that of those below it, and 

 consequently the curve representing the vertical distribution 

 takes the form B, which, from the bottom to within about 

 15 fathoms of the surface, preserves its parallelism to the line 

 of abscissae, but then bends sharply upwards, presenting a 

 \\tll-marked convexity to the origin. This convexity of the 

 curve is the distinctive feature of a vernal distribution of 

 temperature. As the summer advances the temperature of the 

 surface no longer increases at the same rate as before, indeed 

 it tends always more and more to become constant. The heat 

 of the surface layers is, however, always being propagated 

 downwards by conduction, and when the temperature of the 

 surface layer has become nearly constant, it follows that, at 

 some depth a little below the surface, the temperature will be 

 rising more quickly than in the layers above, and this produces 

 a slight bulge in the curve C, representing the distribution. 

 This part of the curve presents a concavity to the origin which, 

 combined with the pronounced convexity below and the less 

 marked convexity above, produces the typical summer distri- 

 bution. When the autumn has set in, and the surface tem- 

 perature falls from day to day, heat is still being propagated 

 downwards by conduction and convection, the curve takes the 

 typical autumnal form D, consisting of a horizontal piece near 

 the surface united to another horizontal piece near the bottom 

 by the summer concavity and the vernal convexity. Hence in 

 the autumn the waters of a deep lake are exposed to all the 

 ditfrri-nt (onditions of the four seasons of the year. In the 

 deeper layers heat is propagated downwards most rapidly in 

 the nr>t half of the month of October. As the winter progresses 

 heat leaves the water so rapidly by the surface that conduction 

 downwards i^ checked and the deeper waters derive but v. i\ 



t In -IK -tit from the summer heat at the surface. 



Observations on iSth August 1885. These observations 



on tin. (1 to the Tarbet basin, and were only carried to a 



depth of 30 fathoms. The weather was perfect for sounding 



operation-, 1>< in- <|uitr calm, so that the steam launch, which 



was used work, remain, d in position without trouble. 



