COAST. I ARTILLERY JOURNAL, - Air Defense Artillery

UNIVERSAL MEASURING CAMERA 443
b. Distortion of the Film Layer. With careful handling of the
film during developing and drying, distortion of the layer is avoided
to a great extent. But careful attention must be given to this in tropical
regions and at high temperatures. The dark room must have good
ventilation, the baths must under certain circumstances be ice cooled
and held at the normal of 15°- 18° centigrade. In drying, the film
should not be hung up, but laid horizontally on glass plates and protected
from, dust.
c. Errors in Measuring Gun Elevation. In general, gun elevations
are measured with an accuracy of -+- 1/16 degree, equal, approximately,
to 4 minutes .of angle. To this must be added the "jump" of
the gun when fired.
For these reasons the angle f3 may be erroneously measured. But
the auxiliary angle a will be influenced by this error in measurement
to the extent of the error of the measure in angle of elevation only
when a is 0 or 180°,that is, when the apparatus stands directly under
the line of the trajectory or behind the gun in direction of the projectile's
flight. In all other positions the influence in error of elevation and
departure will be less. It should, at any rate, be recognized and valued
to the extent of an estimate of its influence on the result.
In order to reduce this error to the minimum, it is advisable to determine
the elevation of the gun with a clinimeter rather than with the
usual appliances, so that then only the error due to jump affects the
result.
d. Measuring Errors of the Angles a and g, and of the Distance B.
As long as the angle of direction a lies between 80° and 120°,the value
of the auxiliary angle 8 varies materially only when the terrain angle
g is also considerably larger than about 5°.
In case a = 90° and g = 0, 8 holds the same value for all elevations
B, namely 90°. The angle a is usually measured from the gun with the
aid of the battery directing arc or the telescopic sight. The accuracy of
this measurement is seldom less than -+- 8 minutes when the measuring
appliance is, as usually the case, provided with 6400 marking lines
for 360° The possible error should in this case, be also calculated and
valued as a possible error whenever it exceeds in amount 0.1%.
The measure of the length B can usually be made with a reasonable
degree of accuracy, since with the normal distances of 30 to 50 meters an
accuracy of -+- 0.08% can be achieved with the customary measuring
rod or tape, even with rapid measurements.
e. Measure Errors in Setting the Camera. The error in setting the
inclination of the objectiye--angle -,,~ranges from : 1h minute to
: I minute of angle.