84 
Proceedings of the Royal Society of Edinburgh. [Sess. 
might exist ; all that can he said is that no such circumstances have been 
demonstrated, and the ready formation of clouds at all heights seems to 
indicate that such circumstances are quite unlikely. Hence the meteoro- 
logist is entitled to infer, as the result of a meteorological though not of a 
physical law, that condensation in the form of cloud, or if necessary of 
rain, will always accompany the reduction of temperature of the air below 
the point of saturation, and the amount of condensation will depend upon 
the reduction of temperature and upon nothing else.* 
These five laws express the special principles with which the meteoro- 
logist must approach the consideration of the circulation of the atmosphere, 
with all its complexities and its perplexities. The rest must depend upon 
the application of the ordinary principles of dynamics and physics to the 
results of observations which indicate the pressure, temperature, and density 
of the air in its actual condition when under consideration. It is my object 
in this paper not to discuss or to justify these principles, but to show how 
far they lead us in the explanation of some of the more general phenomena 
of the atmospheric circulation. 
The form which has been adopted for this communication has been 
chosen for the purpose of drawing a distinction between the inductive, the 
observational, and the deductive aspects of the questions which are treated. 
Just as, in the cases of motion treated in text-books of dynamics, there is 
ample opportunity for discussion as to the form of words which shall be 
used for the laws of motion and the grounds for their acceptance or re- 
jection, starting from the consideration that there never has been an actua] 
example of a body free from the action of force, so, in the case of atmo- 
spheric motion, there is no lack of opportunity for the discussion of the laws 
as here set out, starting from the consideration that no actual case can be 
quoted in which we are certain that the laws are strictly obeyed. And 
further, just as in the case of the dynamics of the heavenly bodies the 
whole subject is reduced to a manageable form by setting out to explain 
the changes of motion and their causes instead of pondering over the 
ultimate origin and cause of the state of motion which exists at any 
particular epoch, so in the study of the circulation of the atmosphere we 
may profitably turn our attention to the changes in the motion related to 
the varying distributions of pressure, and leave for the time being the 
endeavour to give a short answer to the question, “ What is the ultimate 
* The supersaturation of atmospheric air is discussed in Dr Alfred Wegener’s Thermo- 
dynamik der Atmosphare, Leipzig, J. A. Barth, 1911. Humidities, by the hair hygrometer, 
up to 107 per cent, are cited on p. 254 of that work. 
