to +2 percent as regards distance to ground zero, and to +4 degrees as regards angular bearing. 4.2 TIMING CONTROL It is necessary to know the time, relative to detonation, at which each photograph is exposed. This problem was solved in principle by photographing a clock, mounted within the camera, at the same time each exposure is made. The remaining problem is one of synchronization. In most cases the clock was setto within a second or two, and the clock error was noted. However, in two cases the clock ran slow because of low temperatures. In another.case, in which clock error is not noted but is of the order of 3 minutes, the only way to ascertain the time is to work backward from the best data obtained from other cameras. 4.3 MEASUREMENTS FROM THE FILMS It is impossible to specify an exact measurement procedure, since each photograph differs importantly from all others. The analyst takes advantage of every feature that can aid him, primarily in the matter of establishing a gound-reference level. Many such references are possible; sometimes all of them are available and sometimes none. The best reference would be a clearly defined ground zero, such as may be derived from the ellipse representing the water-shock intersection. Other possible referencesare the . horizon, a well-defined condensation ring or skirt surrounding the stem, the edge of the film frame, or the fiducial references in the camera film-plane. Frequently these fiducial marks are missing, because they require sky light to shadow them on the film; artificial illumination would be much more reliable. Having chosen the reference for a given photo, measurements are taken to tie them all together, i.e., horizon to fiducials, shock-wave to condensation ring, and se on. Such data, obtained from a few pictures, may be invaluable for making measurements from other photos in the same series. Thus, measurements may be made from the shock circle for the first few pictures, from a prominent skirt later on, and finally, from the fiducials when they reappear. Measurements are taken from the best reference for the particular picture to the several interesting cloud features. Careful notesare kept explaining the references used and the portions of the cloud to which measurements were taken. These notes are used later in resolving the discrepancies between readings from several cameras. The 35-mm films are read by means of a Hauser microscope comparator or by means of a magnifier and scale on an illuminated table. The comparator provides more precision than is required '(£0.0002 inch on the film) and is rather time consuming, in view of the careful set-up required. The magnifier and scale permit readings to an estimated 40.001 inch, which is more than adequate for these measurements. The choice of instrument is usually made in view of the particular features of the given film. The K-17C films are read by means of a scale on an illuminated table, which is - good to +0.0linch. This is more than adequate for these purposes, since the edge of the irregular cloud usually cannot be defined to better than 0.10 inch on these filma. 4.4 CALCULATIONS Scale factors are derived for dach film as a function oftime from the smoothed airplane position curve and from thc calibrated focal lenyth. Then the appropriate cor~ rections are derived, also as functions of time. These corrections (for earth curvature and refraction, position of the apparent horizon, altitude of a particular skirt used as 18 SECRET ee See neem tiene HY ee oe tamM Aut mERSG ste Acne ~ . TO eerie sem cemme aeemals Setce LRT Aestate see