94 
Proceedings of Royal Society of Edinburgh. 
supposition. Now it is very evident that very many clouds are in 
the process of decay, and their forms can only be explained by the 
processes going on under these conditions. 
This brings me to the second point in this communication, 
namely, the manner in which ripple-marked cirrus clouds are pro- 
duced. The explanation which has generally been accepted of the 
formation of this form of cloud is, that the ripple markings are due 
to the general movements of the air giving rise to a series of eddies 
the axes of the eddies being horizontal, and roughly parallel to each 
other. It is very evident that the air revolving round these hori- 
zontal axes, that is, in a vertical plane, will at the lower part of its 
path be subjected to compression, and at the upper part to expan- 
sion. The result of this will evidently be, supposing the air to be 
nearly saturated with moisture, a tendency for cloudy condensation 
to take place in the air at the upper part of its path, and it is this 
cloudy condensation in the upper part of the eddies that is sup- 
posed to produce the ripple-like cirrus ; each ripple mark indicating 
the upper part of an eddy. One objection I have always felt to this 
explanation is, that it is difficult to imagine that the small amount 
of elevation and consequent expansion and cooling could give rise 
to so dense an amount of clouding as is generally observed. Any 
clouding produced in this way one would expect to be extremely 
thin and filmy. I have for the last few years made frequent 
observations of these clouds, and I have to admit I have never 
once seen them in the process of formation, or seen one appear in a 
clear sky. In all cases that have come under my observation, 
these ripple clouds have been clouds in decay. They are generally 
formed out of some strato-cirrus or similar cloud. When we observe 
these strato-cirrus in fine weather, it will be found that they 
frequently change to ripple-marked cirrus before vanishing. The 
process of their formation would seem to be : the strato-cirrus 
gradually thins away till it attains such a depth, that if there are 
any eddies at their level, the eddies break the stratus cloud up into 
parallel or nearly parallel masses, the clear air being drawn in 
between the eddies. It will be observed that this explanation re- 
quires the eddies, but not to produce the clouding, only to explain 
the breaking up of the uniform cirrus cloud into ripple cirrus. 
One thing which supports this explanation is, that lenticular- 
