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Taking the LovelaTFor accurate work, it is necessary to ascertain the comparative levels of the tops of all of the grade stakes; or the distance of each one of them below an imaginary horizontal plane. This plane, (in which we use only such lines as are directly above the drains,) may be called the “Datum Line.” Its elevation should be such that it will be above the highest part of the land, and, for convenience, it is fixed at the elevation of the levelling instrument when it is so placed as to look over the highest part of the field.
Levelling Instruments are of various kinds. The best for the work in hand, is the common railroad level, which is shown in Fig. 6. This is supported on three legs, which bring it to about the level of the eye. Its essential parts are a telescope, which has two cross-hairs intersecting each other in the line of sight, and which may be turned on its pivot toward any point of the horizon; a bubble glass placed exactly parallel to the line of sight, and firmly secured in its position so as to turn with the telescope; and an apparatus for raising or depressing any side of the instrument by means of set-screws. The instrument is firmly screwed to the tripod, and placed at a point conve~ nient for looking over a considerable part of the highest land. By the use of the set-screws, the plane in which the instrument revolves is brought to a level, so that in whatever direction the instrument is pointed, the bubble will be in the center of the glass. The line of sight, whichever way it is turned, is now in our imaginary plane. A convenient position for the instrument in the field under con sideration, would be at the point, east of the center, marked K, which is about 3 feet below the level of the highest part of the ground. The telescope should stand about 5 feet above the surface of the ground directly under it.
17w Levelling-Rod, (See Fig. 7,) is usually 12 feet long, is divided into feet and hundredths of a foot, and has a
movable target which may be placed at any part of its entire length. This is carried by an attendant, who holds it perpendicularly on the top of the grade-stake, while the operator, looking through the telescope, directs him to move the target up and down until its center is exactly in the line of sight. The attendant then reads the elevation, and the operator records it as the distance below the datum-line of the top of the grade-stake. For convenience, the letterings of the stakes should be systematically entered in a small field book, before the work commences, and this should be accompanied by such a sketch of the plan as will serve as a guide to the location of the lines on the ground.
The following is the form of the field book for the main drain 0', with the levels recorded :
The levelling should be continued in this manner, until the grades of all the points are recorded in the field book.
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 im pcrtant to have recorded in their proper positions, for convenience of reference during the work.
The Depth and Grade of the DralnS.——Having now staked out the lines upon the land, and ascertained and recorded the elevations at 'the difi'erent stakes, it becomes necessary to determine at what depth the tile shall be placed at each point, so as to give the proper fall to each line, and to bring all of the lines of the system into accord. As the simplest means of illustrating the principle on\ which this work should be done, it will be convenient to go through with the process with reference to the main drain 0, of the plan under consideration. A profile of this line is shown in Fig. 21, where the line is broken at stake N 0. 7, and continued in the lower section of the diagram. The topmost line, from “ Silt Basin ” to “ 17,” is the hori~ zontal datum-line. The numbers above the vertical lines indicate the stakes ; the figures in brackets between these, the number of feet between the stakes ; and the heavy figures at the left of the vertical lines, the recorded measurements of depth from the datum-line to the surface of the ground, which is indicated by the irregular line next below the datum-line. The vertical measurements are, of course, very much exaggerated, to make the profile more marked, but they are in the proper relation to each other.
The depth at the silt-basin is fixed at 2% feet (2.50.) The rise is rapid to stake 3, very slight from there to stake 7, very rapid from there to stake 10, a little less rapid from there to stake 11, and still less rapid from there to stake 17.
To establish the grade by the profile alone, the proper
course would be to fix the depth at the stakes at which the inclination is to he changed, to draw straight lines between the points thus found, and then to measure the vertical distance from these lines to the line indicating the surface of the ground at the different stakes; thus, fixing the depth at stake 3, at 4 feet and 13 hundredths,* the line drawn from that point to the depth of 2.50, at the siltbasin, will be 3 feet and 62 hundredths (3.62) below stake 1, and 3 feet and 92 hundredths (3.92) below stake 2. At stake 7 it is necessary to go suficieutly deep to pass from '7 to 10, without coming too near the surface at 9, which is at the foot of a steep ascent. A line drawn straight from 4.59 feet below stake 10 to 4.17 feet at stake 17, would be unnecessarily deep at 11, 12, 13, and 14; and, consequently it is better to rise to 4.19 feet at 11. So far as this part of the drain is concerned, it would be well to continue the same rise to 12, but, in doing so, we would come too near the surface at 13, 14, and 15 ; or must considerably depress the line at 16, which would either make a bad break in the fall at that point, or carry the drain too deep at 17.
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 to~ ward its outlet:
It will be seen that the fall becomes more rapid as we ascend from stake 7, but below this point it is very much
* The depth of 4.13, in Fig. 21, as well as the other depths at the points at which the grade changes, happen to be those found by the computation, as hereafter described, and they are used here f(\‘ illustration.