50 
ABBE. 
fessor Hann wrote an explanatory article in the Zeitschrijt 
of the Austrian Meteorological Society for 1874, showing 
how the laws of thermodynamics apply to the atmosphere. 
This paper was followed by much more elaborate studies and 
a series of valuable publications by others, so that it is now 
easy to apply our knowledge of thermodynamics to the 
atmosphere. A most helpful memoir along this line was 
that by Hertz, in which he gave a very simple diagram 
(known everywhere as the Hertzian diagram of adiabatics 
for the atmosphere) for determining what the condition of 
moist air must be on attaining a given height in the atmos- 
phere. Assuming that it retains its original amount of heat 
during the whole time, his diagram shows very clearly the 
results of the ascent of ordinary clear air up to a level at 
which cloud-formation begins; then to the level at which the 
precipitation is in the shape of frozen water-drops, or hail, 
and above that to the region in which precipitation must be 
in the shape of icy spiculae, or snow. These four stages of 
cooling, viz., the dry stage, cloudy stage, ice stage, and snow 
stage, characterize nearly all the important phenomena of 
the weather. 
In his further applications of thermodynamics Professor 
von Bezold has clarified our ideas by introducing a series of 
diagrams after the manner first taught by Clapeyron. As- 
suming that a unit mass of air mixed with a given quantity 
of moisture rises or falls adiabatically, his diagrams then 
show its condition at any moment by means of curves anal- 
ogous to those used by the steam engineer when he wishes to 
ascertain the condition of the steam in his cylinder and the 
amount of work being done by it. Von Bezold also shows 
how to treat any changes in the air that are not adiabatic, 
although so nearly so that they can be called pseudo- 
adiabatic. Lately a student of von Bezold, Dr. Neuhoff, has 
published a modification of Hertz’s diagram, together with 
elaborate tables, by means of which most problems in the 
formation of cloud, rain, and hail or snow may be very 
easily solved, and with as much accuracy as the present state 
of our knowledge allows. A still more extensive w r ork along 
