By the arrangement adopted, the grade is broken at 3, 7, 10, and 11. Between these points, it is a straight line, with the rate of fall indicated in the following table, which commences at the upper end of the drain and proceeds toward its outlet:
| From Stake, Depth. | To Stake, Depth. | Distance. | Total Fall. | Rate of Fall. Per 100 Ft. | |
| No. 17...4.17 ft. | No. 11...4.19 ft. | 246 ft. | 2.46 ft. | 1.09 ft. | |
| No. 11...4.19 ft. | No. 10...4.59 ft. | 41 ft. | 82 ft. | 2.00 ft. | |
| No. 10...4.59 ft. | No. 7...4.47 ft. | 91 ft. | 2.49 ft. | 2.83 ft. | |
| No. 7...4.47 ft. | No. 3...4.13 ft. | 173 ft. | 96 ft. | 56 ft. | |
| No. 3...4.13 ft. | S. Basin 2.25 ft. | 186 ft. | 3.47 ft. | 1.87 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[pg 109] 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]
| Fall. Feet and Decimals. | Depth from Datum Line. | ||||||
| No. of Stake. | Distance Between Stakes. | Per 100 Feet. | Between Stakes. | To Drain. | To Surface. | Depth of Drain. | Remarks. |
| Silt Basin. | 20.70 ft. | 18.20 ft. | 2.50 ft | ||||
| C. 1. | 82 ft. | 2 ft. | 1.64 ft. | 19.06 " | 15.44 " | 3.48 ft | |
| C. 2. | 39 ft. | do. | .78 ft. | 18.28 " | 14.36 " | 3.83 ft | |
| C. 3. | 65 ft. | do. | 1.30 ft. | 16.98 " | 12.85 " | 4.13 ft | |
| C. 4. | 51 ft. | .56 | .28 ft. | 16.70 " | 12.18 " | 4.52 ft | |
| C. 5. | 43 ft. | do. | .24 ft. | 16.46 " | 11.79 " | 4.67 ft | |
| C. 6. | 47 ft. | do. | .26 ft. | 16.20 " | 11.69 " | 4.51 ft | |
| C. 7. | 32 ft. | do. | .18 ft. | 16.02 " | 11.55 " | 4.47 ft | Silt-Basin here. Made deep at Nos. 7 and 10 to pass a depression of the surface at No. 9. |
| C. 8. | 41 ft. | 2.83 | 1.16 ft. | 14.86 " | 11.37 " | 3.49 ft | |
| C. 9. | 12 ft. | do. | .34 ft. | 14.52 " | 11.06 " | 3.46 ft | |
| C.10. | 38 ft. | do. | .99 ft. | 13.53 " | 8.94 " | 4.59 ft | |
| C.11. | 41 ft. | 2.00 | .82 ft. | 12.61 " | 8.52 " | 4.19 ft | |
| C.12. | 41 ft. | 1.09 | .44 ft. | 12.27 " | 7.86 " | 4.41 ft | |
| C.13. | 41 ft. | do. | .44 ft. | 11.83 " | 7.70 " | 4.13 ft | |
| C.14. | 41 ft. | do. | .44 ft. | 11.39 " | 7.39 " | 4.00 ft | |
| C.15. | 41 ft. | do. | .44 ft. | 10.95 " | 7.06 " | 3.89 ft | |
| C.16. | 41 ft. | do. | .44 ft. | 10.51 " | 6.73 " | 3.88 ft | |
| C.17. | 41 ft. | do. | .44 ft. | 10.07 " | 5.90 " | 4.17 ft |
Note.—The method of making the foregoing computation is this:
1st. Enter the lettering of the stakes in the first column, commencing at the lower end of the drain.
2d. Enter the distances between each two stakes in the second column, placing the measurement on the line with the number of the upper stake of the two.
3d. In the next to the last column enter, on the line with each stake, its depth below the datum-line, as recorded in the field book of levels, (See page 105.)
4th. On the first line of the last column, place the depth of the lower end of the drain, (this is established by the grade of the main or other outlet at which it discharges.)