come under more intensive investigation re- 
cently, they have not yielded a body of knowl- 
edge that has replaced the mixing-length 
theories in the applications, 
The statistical theories of homogenous tur- 
bulence and turbulent diffusion originated in 
the pioneering work of Taylor (25, (26), In 
addition to the work of Taylor (27) and many 
others, important fundamental developments of 
the mathematical theory of turbulent diffusion 
for both the single- and the two-particle 
fomulations were set forth in two papers by 
Batchelor (2, 3), The statistical theories of 
turbulence and turbulent diffusion have been 
the subject of intensive investigation since 
the end of World War II, 
In U.S, Department of Agriculture research 
in this area, a problem to be given attention is 
the turbulent diffusion in boundary layers, 
owing to its importance in determining the 
structure of surface deposits, Studies of bound- 
ary deposits of particulate matter encountered 
in such situations will be made with the aid of 
the previously described particle analyzer and 
a fluorescent racer method, 
A continuous scanning technique is under 
development for use with fluorescent tracers 
to determine whether a conceptually useful 
characterization of an aerosol deposit distri- 
bution may be obtained, Visible fluorescence 
from the tracer material, which is excited by 
a beam of ultraviolet radiation, is collected 
by a microscope as a test Surface moves past 
its objective at a constant rate, Appropriate 
primary and secondary optical filters suited 
to the ultraviolet source and the tracer are 
included in the optical path, The continuous 
scanning apparatus is not particularly sensitive 
to individual particles but responds in an 
integrated manner to the number of particles 
instantaneously in scan, The fluorescence is 
sensed by a photomultiplier system and the 
output signal is recorded on magnetic tape. 
Since the studies to date have been limited to 
(statistically) homogeneous deposits, an ap- 
parently stationary random signal is recorded 
as an output. Therefore, the signals will be 
analyzed by the techniques of generalized 
harmonic analysis, which are suited to such 
applications, just as is done in the analysis of 
turbulence, Both digital and analog techniques 
will be available for data processing. 
206 
It is likely that two types of continuous 
scanning will be necessary; (1) A low magnifica- 
tion system intended to scan over relatively 
large distances and larger scale deposits and 
(2) a high magnification microscope scanning 
system for more detailed investigations of 
small-scale deposits on limited areas, In ad- 
dition, it is reasonable that discrete results 
obtained from uniformly spaced samples of 
aerosol deposits measured by standard chemi- 
cal methods will be amenable to similar 
analyses by digital computer, The particle 
analyzer will be employed in more detailed 
studies, with ordered sequences of readings 
being subjected to a digital analysis in order 
to examine spectra or correlations, By means 
of suitable responsive anemometry equipment, 
studies will be made of the associated tur- 
bulence field to examine the relationships 
between turbulence and deposit structures, 
It is appropriate to speculate about some of 
the implications of particle diffusion studies 
being carried out in several places with re- 
spect to pesticide application, For example, 
(1) a more meaningful and useful description 
of fluid transport processes near plants, in 
the atmosphere, and in application devices 
should be possible; (2) studies of pesticidal 
drift should be greatly aided, and the ability 
to predict drift distribution may be enhanced; 
(3) the ability to economically describe dis- 
tribution of pesticide material on surfaces 
down to perhaps the microscale likely will be 
greatly developed; (4) deeper insight may be 
gained into relationships among force fields, 
turbulence and air flow character, and deposit 
structure; and (5) it should be possible to use 
even more efficiently deposit and residue 
analyses and to state more firmly about spacing 
of samples in making such measurements, 
ENGINEERING 
Industrial Application 
Although industry engages in basic as well 
as applied and development engineering, it 
must, as one industrial engineer puts it, 
carefully consider for each machine a char- 
acteristic that may be described as its ''degree 
of practical accuracy" (10), The machine must 
