FROM STAKE, TO STAKE, DISTANCE. TOTAL FALL. RATE OF DEPTH. DEPTH. FALL. PER 100 FT. No. No. 246 ft. 2.46 ft. 1.09 ft. 17...4.17 11...4.19 ft. ft. No. No. 41 ft. 82 ft. 2.00 ft. 11...4.19 10...4.59 ft. ft. No. No. 91 ft. 2.49 ft. 2.83 ft. 10...4.59 7...4.47 ft. ft. No. No. 173 ft. 96 ft. 56 ft. 7...4.47 3...4.13 ft. ft. No. S. Basin 186 ft. 3.47 ft. 1.87 ft. 3...4.13 2.25 ft. ft. It will be seen that the fall becomes more rapid as we ascend from stake 7, but below this point it is very much reduced, so much as to make it very likely that silt will be deposited, (see page 91), and the drain, thereby, obstructed. To provide against this, a silt-basin must be placed at this point which will collect the silt and prevent its entrance into the more nearly level tile below. The construction of this silt-basin is more particularly described in the next chapter. From stake 7 to the main silt-basin the fall is such that the drain will clear itself. The drawing of regular profiles, for the more important drains, will be useful for the purpose of making the beginner familiar with the method of grading, and with the principles on which the grade and depth are computed; and sometimes, in passing over very irregular surfaces, this method will enable even a skilled drainer to hit upon the best adjustment in less time than by computation. Ordinarily, however, the form of computation given in the following table, which refers to the same drain, (_C_,) will be more expeditious, and its results are mathematically more correct.(16)