of Edinburgh, Session 1885 - 86 . 
405 
been carefully and repeatedly compared with each other, and with 
a Kew corrected standard. The temperatures given are all in 
terras of the Kew standard. As a rule, the same thermometer has 
been sent to the same depth. Further, the same sounding line was 
used on all occasions. 
The results are collected in tables, and in some cases they are 
represented graphically by curves. 
If we represent the distribution of temperature graphically by a 
curve, having depths measured along the horizontal line of abscissae 
and temperatures along the ordinates, the winter distribution will 
be 'represented by a straight line parallel to the line of abscissae, such 
as A. As the spring advances and the meridian altitude of the sun 
daily increases, the temperature of the surface rises rapidly. The 
heat received at the surface is, during this season, propagated down- 
wards, chiefly by conduction, which, in water, is a comparatively 
slow process, 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 well- 
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 con- 
stant, 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 
distribution. A^hen the autumn has set in, and the surface tempera- 
ture falls frofin 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 
