362 
teors, by photographic techniques and by combin- 
ing photographic and radio techniques. 
2. Improved theoretical evaluations of the drag co- 
efficient y, the heat-transfer coefficient \, and the 
luminous efficiency factor 7. 
3. Experimental determinations of y, \, and 7m by 
wind-tunnel, ballistic-range, or rocket procedures. 
4, Studies of the physical and chemical nature of 
meteoroids through the study of micrometeorites. 
The U. 8S. Naval Bureau of Ordnance has made 
possible the continued analysis of Harvard meteor data 
at the Center of Analysis at the Massachusetts Institute 
of Technology, carried out by Dr. L. Jacchia under the 
general direction of Dr. Z. Kopal and with advice 
from the writer. Meanwhile the Bureau of Ordnance, 
via the Harvard Meteor Ballistic Project, has made 
possible the establishment of meteor-observing stations 
in New Mexico under the direction of the writer [91]. 
This program includes the procurement of Super- 
Schmidt Meteor Cameras being made by the Perkin- 
Elmer Corporation from optical designs by Dr. J. G. 
Baker and with critical glass produced by the U. 8. 
Bureau of Standards. These cameras, of focal length 8 
inches, aperture 12 inches (optically F/0.66), and field 
of diameter 52°, should satisfy requirement (1) above, 
particularly if radio equipment can be operated simul- 
taneously nearby. Requirement (4), the study of micro- 
meteorites, will also be undertaken by the Harvard- 
Bureau of Ordnance program. 
The cooperation of the Naval Ordnance Laboratory 
at White Oak, Maryland, has been obtaimed toward 
the fulfillment of requirement (3), particularly the 
determination of the heat-transfer and drag coefficients 
for a model meteoroid made from a substance of low 
melting point, such as COs, in a high-velocity wind- 
tunnel. 
It is hoped that the interest of other scientists and 
research groups may be roused with respect to the 
various problems mentioned in requirements (2), (8), 
and (4). The writer is of the opinion that the poten- 
tialities of the photographic-meteor approach to upper- 
atmospheric research have by no means been exploited 
fully and that rich returns lie ahead. Besides the in- 
creased precision possible in density determinations to 
an altitude of 120 km or more, there are the correlations 
with latitude and season, as well as conceivable synoptic 
air-mass and other correlations. A start is just being 
made on the determination of winds in the region from 
50-100 km by photographic meteors. Close cooperations 
with radio techniques (next section) should produce 
extremely important results concerning dissociation, 
ionization, recombination, absorption of quanta, compo- 
sition, turbulence, and other processes in and near the 
E-layer of the ionosphere. 
RADIO METEORS 
The progress in radar techniques for observing the 
ion columns produced by meteors has been so active 
since Skellett’s suggestion [79, 80] in 1932 that this 
review can touch on only a few points of major interest. 
The reader, for detailed enlightenment, must refer to 
THE UPPER ATMOSPHERE 
survey articles, such as those by Hey [26], Lovell [45], 
and Herlofson [25], or to the original papers. Even the 
present short account will show, however, that the 
future possibilities of these electronic techniques are 
truly enormous for the study of the upper atmosphere 
to a height of at least 130 km via meteoric phenomena. 
The basic principle in the radio observation of me- 
teors involves the “reflection” of high-frequency radio 
waves from the ion column produced by a meteoroid. 
Pierce [70] suggested that the column should produce 
maximum “reflection” for normal incidence of the radio 
beam. The ion columns tend to be more intense in the 
neighborhood of the E-layer; hence the geometrical 
relations limit the radar observation of a given meteor. 
It is found that there is by no means a one-to-one 
coincidence between the visual and radio meteors unless 
the geometrical conditions are well satisfied. 
Two basic electronic techniques are used for observing 
meteors by radio. The first involves the transmission of 
radio pulses a number of times per second, the range 
(distance) being measured by the time lag of the re- 
flected pulse packets—the standard radar technique. 
The second depends upon continuous-wave transmis- 
sion, the Doppler principle providing a modulation at 
the receiver by the beating between the ground wave 
and the reflected wave. 
The pulse-packet technique, first used for ionospheric 
research by Breit and Tuve [8], has been the chief tool 
for radio meteor work until recently. The research 
previous to 1945 was mostly exploratory, finally estab- 
lishing the reality and general character of transient 
echoes from meteor ion columns. The chief workers in 
this period were Skellett [79, 80], Schafer and Goodall 
(74, 75], Appleton, Naismith, and Ingram [1], Pidding- 
ton [2], Pierce [70], Eckersley [14], and Farmer [15]. In 
1945 Hey and Stewart [28] began their observations 
with modified radar equipment operating at a wave 
length of 4-5 m and established the existence of ex- 
tensive daytime meteor activity. They recorded range 
versus time to measure the velocity of the 1946 Gi- 
acobinids by the fast-moving echo preceding the main, 
more persistant, echo. 
Lovell [45] and his colleagues at Manchester have 
since made more extensive meteor observations with 
similar basic equipment, operating for the most part 
near a wave length of 4 m but also near 6 and 8 m. As 
a part of this research project Clegg [11, 12] developed 
a novel and effective method for measuring the radiants 
of meteor showers by a single radar. A narrow beam 
antenna is used to record the frequency and ranges of 
meteors at various orientations of the radiant (time) 
and the antenna. The spacial direction of the radiant 
can then be deduced on the assumption, proven by 
Lovell, Banwell, and Clegg [47], that an ion column 
gives the strongest echo when the radar beam is directed 
perpendicularly to it. By this technique, Clegg, Hughes, 
and Lovell [13] observed a number of daylight radiants 
from May to August 1947 and, with Aspinall [4], con- 
firmed their reoccurrence in 1948. 
In close coordination with Clege’s work, Ellyett and 
Davies [19] developed an ingenious method for measur- 
