MANUAL OF TIDE OBSERVATIONS 39 



102. When a suitable place can be found, it is in general desirable 

 to set the face of the bench mark disk horizontal for convenience in 

 holding the leveling rod, but there should be sufficient clearance 

 above the bench mark for the rod used in the first-order leveling, 

 which is approximately 11 feet long. Sometimes greater permanency 

 can be secured by placing the bench mark in the vertical wall of a 

 building. In this position the short horizontal line through the 

 center of disk becomes the bench mark and a graduated tape may 

 be substituted for the rod in making the leveling connections. A 

 bench mark in a vertical wall should be several feet above the ground 

 for convenience in holding the tape used by the leveling party. Care 

 should be taken to set the disk with the reference line horizontal. 

 The bench mark is to be countersunk with its face flush with the 

 wall, and when set in a conspicuous place special care must be taken 

 to have the work neatly done so that the building will not be 

 defaced. 



103. Bench marks in rock. — Rocky outcrops and boulders in areas 

 not likely to be disturbed for many years make satisfactory locations 

 for bench marks. If a boulder is used the bottom should extend far 

 enough below the surface of the ground so that it will not be affected 

 by action of frost. The standard disk should be cemented in place 

 with its face horizontal- and countersunk. 



104. Bench marks in concrete. — Where a suitable building or 

 natural rock is not available, the bench mark disk may be set in a 

 concrete monument built in the place desired. The monument should, 

 extend not less than 3 feet below the surface of the ground, and 

 in localities having severe winters the depth should be sufficient 

 to withstand frost action. In general the top will extend several 

 inches above the ground in order that it may be readily found when 

 needed. The base should have a cross section greater than the top 

 and there must be no projecting edges which might provide leverage 

 points for frost attion. If the monument is in the form of a frustum 

 of a cone or pyramid, the sides should have a batter of not less than 

 1 inch, to the foot. A convenient shape for making the concrete 

 monument is a series of square blocks from 8 to 12 inches thick, the 

 bottom block being about 18 inches square and successive blocks of 

 diminishing cross section with the top block about 12 inches square. 



105. It is important in making the concrete that all materials be 

 clean and thoroughly mixed before adding water. It should not be 

 too wet and should be well tamped into the mold. When using 

 rough aggregate the proportion should be about 1:3:5, the upper 

 part of the mass to be of a richer mixture. When only sand and 

 cement are obtainable the proportion of 1 part of cement to 3 parts 

 of sand should be used for the lower part of the mass and 1 part of 

 cement to 2 parts of sand for the upper part. To prevent rapid 

 drying of the concrete, its surface should be covered with dampened 

 paper or cloth or wet seaweed held in place with earth. 



106. Number of bench marks. — To insure against loss of datum, 

 there should be a number of bench marks widely scattered but in 

 general within a radius of 1 mile from the tide station. For all 

 series of tide observations, no matter how short, there should be a 

 reference to at least three substantial bench marks. The minimum 

 requirement for a series of observations covering a year or more is 



