LETTERING OF THE STAKE. DEPTH FROM DATUM LINE. Silt Basin 18.20 C 1 15.44 C 2 14.36 C 3 12.85 C 4 12.18 C 5 11.79 C 6 11.69 C 7 11.55 C 8 11.37 C 9 11.06 C 10 8.94 C 11 8.52 C 12 7.86 C 13 7.70 C 14 7.39 C 15 7.06 C 16 6.73 The levelling should be continued in this manner, until the grades of all the points are recorded in the field book. [Illustration: Fig. 21 - PROFILE OF DRAIN C.] Fig. 21 - PROFILE OF DRAIN C. Horizontal Scale, 66 ft. to the inch. Vertical Scale, 15 ft. to the inch. 1 to 17. Numbers of Stakes. (82) etc. Distances between Stakes. 18.20 etc. Depths from _datum-line_ to surface. 2.50 etc. Depths of ditch. 20.70 etc. Depths from _datum-line_ to drain. If, from too great depression of the lower parts of the field, or too great distances for observation, it becomes necessary to take up a new position with the instrument, the new level should be connected, by measurement, with the old one, and the new observations should be computed to the original plane. It is not necessary that these levels should be noted on the map,—they are needed only for computing the depth of cutting, and if entered on the map, might be mistaken for the figures indicating the depth, which it is more important to have recorded in their proper positions, for convenience of reference during the work.