148 Proceedings of the Royal Society of Edinburgh. [Sess. 
turbulent layers full of eddies, though the tops are approximately at one 
level. The eddies probably carry heat downwards from the level of the 
tops of the clouds to some distance below them. This would cause the 
adiabatic temperature gradient within the clouds, and the reversed gradient 
above them, which are characteristic of these clouds; and in some cases 
this may be the only cause of these phenomena. In the case of the more 
marked reversed gradients other factors are probably of importance, and 
the two layers of air may often be of widely different origin, as suggested 
in my paper. The arguments concerning the formation of reversed 
temperature gradients with west winds remain unaltered if the words 
“ mixing due to turbulent motion ” be substituted for “ convection.” In 
passing from a turbulent region to an undisturbed stratum, it is to be 
expected that there should be an increase of the west to east component of 
the wind velocity, and this is consistent with what I have observed. In 
some cases the difference of wind velocity may cause the turbulence, and in 
these cases the clouds might be said to be due to warm air flowing over 
cold air. 
V 
( Issued separately April 30, 1917.) 
