curve and, of the two curves, the data for the sand curve were more extensive and fitted more closely to a curve of low degree (see Fig. 3). However, there were a few curves that departed Significantly from the sand curve and those Figure 3. are also shown in The decay correction factor was determined by divid- ing the value on the sand curve for December 1 by the value on the sand curve for the day the sample was counted. The range of these factors was from 0.68 for November 24 to 1.51 for December 12. The samples from the pre-test collections were not cor- rected for decay since the change in counting rate during the perfod the samples were counted was slight. factors would have been about 1 percent. Maximum correction Por differences be- tween pre and post shot decay curves see Figure 3. Self-absorption. for self-absorption. This year sample counts were corrected In 1949 no correction for self-absorp- tion was determined but an attempt was made to keep the ash on the plate thin and constant in amount. Although it was recognized that the types and the proportions of isotopes varied from sample to sample, the decay and mass absorption curves (see Figs. 3 and 4) indicate that the sand sample approximates the mean of all the curves. Hence the same sand that was used to determine the correction factor for decay was also used to determine the self-absorption correction factor. A few of the actual values, based on the total weight of ash on 14-inch plate, are as follows: - 1h - 100 mg, 2.6;