316 THE FUTURE OF ARID LANDS 
Schaefer (5, 6) reported that some soil and dust particles have 
nucleating properties. Bowen (1) suggested a relationship between 
rainfall and variations in meteoritic dust although the evidence on 
this point is circumstantial in nature as pointed out by Neumann 
(4) and Swinbank (g). Without question, more direct measure- 
ments of the quantities concerned are needed before most of the 
theories proposed can be confirmed or rejected. 
Collecting such data is expensive and no program for indis- 
criminate collection of such data is recommended. Initial efforts 
are likely to be confined to those areas where the role of nuclei in 
the rain-making process seems to be of greatest importance or 
interest for one reason or another. As more data become available 
and are examined, unexpected relationships will be uncovered, 
some of which will be fortuitous and therefore misleading. There- 
fore a program for collecting the data should also include plans 
for objective and unbiased evaluation of the data, but these plans 
must be flexible enough to permit promising clues to be recognized 
and followed-up effectively. The methods of sequential analysis 
used successfully in other scientific fields might be used here. Maxi- 
mum effectiveness in this program requires the coordinated efforts 
of the climatologist, meteorologist, cloud physicist, chemist, hy- 
drologist, statistician, and astronomer. 
Analysis and Interpretation of Field Data 
If the collection of the field data is supplemented by laboratory 
experiments and studies of physical theories, it should be possible 
to estimate the relative importance of vertical motion and the 
supply of condensation and freezing nuclei in the production of 
natural rain. Data on the natural variation of these particles could 
be correlated with rainfall occurrence, cloud structure, and electri- 
cal effects. These data might shed new light on Bowen’s meteoritic 
dust hypothesis or reveal other important effects that influence 
precipitation. For example, the role played by atmospheric elec- 
tricity is not understood, but other pioneers in cloud modification 
including Gunn (2) of the Weather Bureau and Vonnegut (11) are 
giving careful attention to this subject. 
Because of the absence of a complete physical theory on the 
