Refraduced from #239 DLNJrevs 8 ~28 =5 3 micrometer scale is recorded, the alignment light moved off line in the opposite direction, by means of the tangent screw, and the operation repeated, After a total of sixteen readings has been taken, eight from each side of the line, the alignment light is adjusted to set the micrometer scale at the mean of these sixteen readings. A plumb bob is then suspended from a scribe mark centering the light slit and a reference mark is made on the monument cap. The instrument is then moved to this newly established point, the alignment equipment to the next location and the operation repeated, The complete operation establishing and checking all intermediate points should be performe ed on three successive nights or until results indicate that align-= ment has been established to specified tolerances. The second alignment should be made in the reverse direction, Complete records should be made of each operation including the alignment micrometer readings and any variations found in the checks made on successive nights. mace 62 points at the extremities of the line for vertical control, the effects of curvature of the earth's surface and refraction must be eliminated, As the effects of this refraction cannot be accurately predetermined, it is believed that greater refinement in vertical alignment can be obtained by establishing a level line and compensating for curvature of the earth's surface than by attempting a direct line of sight be-= tween working points, A series of bench marks should be established along the line at approximately 500 foot intervals. A precise level and Invar rods are available at the jobsite and are satisfactory for this purpose. procedure is, starting at one end, set up the each successive pair of bench marks and carry to the opposite end of the line andreturn, by the bench marks to the initial mark. Extreme The level midway between the elevations through the same method, through care should be taken in leveling of the instrument and the plumbing and reading of the rod, The effect of refraction and instrumental error will be negligible if the rod readings are taken with the instrument set up midway between each pair of bench marks. The operation is repeated and an additional check made between any two bench marks where an obvious discrepancy is found. The closing error in each circuit should not exceed 0.017 foot times the square root of the distance in miles. The adopted elevations of the bench marks are the mean elevations of the two circuits after the closing error has been porportioned through the circuits. When satisfactory elevations of the bench marks have been established, the relation is computed between the level line following the curvature of the earth's surface and a straight line tangent to this curvature at the mid point of the line. This correction is based on the radius of curvature of the earth's surface at the latitude of the observer —— al ,