THE DEVELOPMENT OF METEOROLOGY. 
49 
oirs on fluid resistances, including the first solutions of prob- 
lems involving vicosity or internal friction of fluids. To 
Prof. Joseph Reynolds we owe some beautiful experiments 
showing how the motion of a fluid changes from a laminar 
flow to a vortical flow whenever the excess of internal energy 
amounts to a very small limit, and vice versa. 
To Willy Wien, a pupil of Helmholtz, we owe the develop- 
ment of problems relating to vortex and wave motions in the 
earth’s atmosphere. To Professor Pockels, another pupil, 
we owe a very ingenious memoir on the influence of mountain 
slopes in forcing moist air to ascend and form clouds and 
rain. It is the presence of aqueous vapor in our air and the 
consequent thermodynamic complications that necessitates a 
combination of hydrodynamics with thermodynamics and 
leads to still more complex mathematical problems, whose 
solution is absolutely necessary if we would understand the 
formation of clouds and rain. In this branch of study we 
have an instructive memoir by Briliouin, in which he is able 
to explain in a general way the formation of many types of 
clouds or layers of clouds as due to the mixture of masses of 
air having different degrees of temperature and moisture. 
The appearances of the clouds have been most carefully ob- 
served and recorded for two centuries, but the ability to 
learn what they can teach us has only become possible within 
the past thirty years. 
The first step in the application of thermodynamics to 
meteorology was undoubtedly taken by Espy in 1822, when 
lie stated that the cooling due to the expansion of air ascend- 
ing into regions of lower pressure caused the formation of 
clouds and the lower temperature of the air of the upper 
strata. However it was soon found that the cooling is not 
due to the expansion as such, but to the work done by ex- 
pansion against atmospheric pressure. This general ex- 
planation was accepted by French physicists in 1839, 
but was given greater precision by Sir William Thom- 
son in 1864, and Peslin in 18b9; it was satisfactory 
to American, English, and French students, but seems 
not to have been accepted in Germany until Pro- 
