encountered infrequently.
For the Pinon shot the images will be approxi-
mately hemispherical.
The fireball edge will be only moderately sharp,
fsy--e
because of scattering in the 20-mile photographic path; scattering may
ene
also result in poor exposures of low contrast.
Furthermore, natural
clouds will probably obscure portions of the fireball, since it will grow to
a considerably greater height than the normal low-lying cloud cover.
These
factors introduce the necessity for exercising some degree of judgment in
measuring the fireball diameters.
The orientation of the film on the comparator stage is such that the
images will appear to be on their sides, with the horizon vertical.
The
image of the selected picture is centered, making use of the micrometers
and the rotational adjustment, so that the base of the fireball coincides with
the vertical diameter of the circular grid pattern.
It will undoubtedly be
necessary to adjust the illumination for both stages, and in some cases to
switch the upper stage light on and off to aid in locating the fireball edge
in reference to the grid.
The observercan usually bracket the average diameter quickly by
noting the greatest and the least circles of contact.
These limits can be
narrowed by running the eye around the intermediate circles, finally interpolating to arrive at a best estimate of the average or effective diameter.
Experience ‘and practice are required to make this determination quickly
and consistently, and without undue fatigue.
It is always best, in our
experience, to have each picture measured by several persons; in the
event of disagreement between them a short discussion usually suffices to
reach a joint reading agreeable to everyone.
Before moving the film to read another frame, it will be necessary
to measure with the micrometer the distance from the film-speed mark to
the trailing sprocket hole.
These measurements only need to be precise
to about 0.03 inches, or one-tenth of a frame, anda visual estimate may
suffice instead of a measurement.
They will enable the calculation of the
time for the particular picture.
25.
Calculation of Total Yield
Measurements and calculations are best worked out in tabular, systematic form.
We usually record all measurements and calculations on
vellum, which may be reproduced in any desired quantities easily and
ree
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