312 
Sets. 
Fic. 3—Blower and steam generator for produc- 
ing drops 
ciple to be specially adapted for each series of 
experiments. 
The remaining parts of the tunnel serve only to 
allow the air to expand slowly and to bring the 
circulation back to the blower. 
ROLAND LIST 
Fig. 4—The injection of humidity and ice- 
forming nuclei 
The whole plant is supervised and run from the 
control panel (marked as | in Figure 1). Here the 
object under investigation can be directly ob- 
served at the same time that all the values which 
an be regulated are set on the panel. All the 
important values, whether of temperature or 
some other electrical scale, are simultaneously 
recorded on three compensation recorders. The 
apparatus for regulating temperature is housed 
independently; the gages for measuring pressure 
and humidity are likewise located close by the 
measuring section (Fig. 5). 
From this description it will be clear how, in 
the part of the tunnel below the measuring sec- 
tion, a cloud is continuously produced more or 
less subcooled according to need; it does not cir- 
culate more than once, however, as drops of water 
are deposited at the latest in the air cooler and 
any particles of ice on the electro-filter. 
The whole tunnel is constructed of separate 
components, which can easily be taken apart and 
changed. The main substance from which it is 
built is a cellular, slightly spongy plastic called 
Polystirol; it combines lightness of weight (y = 
20 kg/m*) with insulating properties similar to 
those of cork, and has by comparison the advan- 
tage of an even lower thermal capacity relative to 
the unit of volume. These characteristics all guar- 
antee minimal losses of heat with minimal ther- 
