1104 
should be standardized among observers and tech- 
nicians as far as is practicable. 
Several types of volumetric sampling devices (such 
as the Owens dust counter) are available in some quan- 
tity, but most of them handle an insufficient volume of 
air to be adaptable to aerobiological mvestigation. 
While Durham [8] has reported the development of an 
apparatus which impinges the particles from a meas- 
ured quantity of air on a slowly moving slide, thus 
affording both a record of organisms and of the volume 
of air sampled throughout the 24-hour period, the 
results of collections by this method have not been 
reported upon in detail. In a study of various types of 
air filters for trapping microorganisms, DallaValle and 
Hollaender [5] found that the relative efficiency of a 
viscous impinging surface tested was approximately 70 
per cent. They report that the highly efficient Green- 
burg-Smith impinger, used as a dust-sampling appa- 
ratus by the U. 8. Public Health Service, is not par- 
ticularly adaptable to aerobiological work. It is the 
opinion of the writer, however, that the efficiency of the 
collector, itself, is not nearly so important as the con- 
dition that the relative efficiency be a known factor 
and approximately a constant. 
For sampling the upper atmosphere the collecting 
equipment should be compact, light in weight (par- 
ticularly if it is to be supported aloft by balloon, para- 
chute, or special atmospheric sounding device), and 
fully automatic in operation; it should be designed to 
exclude the possibility of contamination! and to pro- 
vide for the multiple sampling of measured volumes of 
air at known temperatures (and humidities) between 
specified small pressure intervals (or at known alti- 
tudes). 
Proctor [23] has perfected an apparatus for upper-air 
sampling by aircraft which, in addition to securing 
operation independent of the pilot, simultaneously re- 
cords certain meteorological data. In addition, variation 
in air flow is recorded to permit comparative counts in 
known volumes of air. As in his previous models of 
collecting instruments, provision is made for photo- 
micrography and subculture of the microorganisms with 
the exclusion of contamination or cross innoculation of 
the separate samples. 
As would be expected, the problem of collecting and 
identifying the larger organisms and insects is simpler 
than the sampling and counting of air-borne microor- 
ganisms. The most common methods for collecting 
these larger forms at or near the surface involve the use 
of nets, flight or baited traps of some sort, or simply 
entail the visual counting of insects on the wing or as 
they alight on some object. None of these methods, 
however, serve to give the true population of organisms 
in the air at any given time, although it is possible, 
under certain conditions, to convert flight-trap col- 
lections into rough volumetric terms. 
Kites and balloons were naturally the earliest devices 
1. The element of contamination is an important considera- 
tion when sampling the upper atmosphere, the air over the 
oceans, or in other situations where population densities of 
microorganisms are low. 
BIOLOGICAL AND CHEMICAL METEOROLOGY 
used for sampling insect populations in the upper air, 
but few of the results of collections by either method 
appear to have been published. The most successful 
collections appear to have been obtained through the. 
use of aircraft. Perhaps the most extensive study made 
of the insect population of the atmosphere was conducted 
at Tallulah, La., during a five-year period between 1926 
and 1931 by the Bureau of Entomology and Quaran- 
tine of the U.S. Department of Agriculture. The results 
have been reported upon in considerable detail by 
Glick [9, 10]. In these collections some 28,739 speci- 
mens were taken, from altitudes as low as 20 ft above 
the ground to as high as 15,000 ft with specially de- 
signed traps fitted to the wings of several types of air- 
craft. The results are given in volumetric terms (average 
volume of air per insect) although desirable details 
concerning the testing and calibration of the collecting 
instrument are lacking. It is unfortunate that similar 
data are not more generally available for other regions. 
Methods and Problems of Identifying Organisms 
and Locating Sources. Perhaps the greatest difficulty 
that confronts the aerobiologist in sampling the at- 
mosphere arises from the fact that many species of 
organisms are so similar in appearance that important 
differences which serve to separate species defy de- 
scription or pictorial representation. Even in the cases 
of some economically important plant pathogens, the 
spores cannot be identified by morphological charac- 
teristics alone. In the cases of other forms, such marked 
variations in size, shape, and septation exist within 
the species that identification is impossible unless many 
spores are examined. Because it is certain that many 
closely related species within the same genus differ 
greatly in cultural characteristics, allergenic or bio- 
chemical effects and pathogenicity, the solution to the 
problem of rapid and precise identification of organisms 
is an essential step toward the full development of an 
applied aerobiology. 
The greatest difficulty that confronts the meteorolo- 
gist in his investigations of the local and long-distance 
dissemination of organisms lies in his lack of knowl- 
edge concerning the exact (or even approximate) sources 
of the collected specimens. In the case of the more com- 
monly collected spores and pollens, the parent plants 
or fungi may be so universally distributed that it is 
wasted effort even to attempt speculation as to the 
origin of the air-borne forms. In the case of bacteria 
in the atmosphere the problem is even more difficult, 
for here the entire earth’s surface (including the oceans) 
must be considered as a potential source. It is un- 
fortunate for the meteorologist that the organisms most 
frequently collected and identified are those whose 
sources are most cosmopolitan. 
It is true that the surface of the earth itself can be 
considered the source of specimens collected in the 
upper atmosphere but this is far too broad a classi- 
fication of origin to allow anything beyond gross an- 
alyses of the atmospheric factors involved in the vertical 
and horizontal transport of organisms. By the same 
line of reasoning, the shore line of a large body of 
water may be considered a line source for land forms 
