MANUAL OF TIDE OBSERVATIONS 37 



as desired. In doing this, care must be taken to hold the counterpoise 

 drum against the pull of the counterpoise weight and also to press the 

 float wire against its drum to keep it from springing off as the wire 

 slackens. 



95. Attaching tension weight. — ^In general it is best to have the 

 tension weight operate from a movable pullej'^ (fig. 17) to reduce the 

 Tate of fall. When thus suspended, the weight will descended on an 

 average about 1 foot a day but the rate will vary with the amount of 

 paper on the receiving roller. Place the weight with pulley attached 

 on the floor at the lowest limit of its motion. After installing the 

 necessary fixed pulleys in the ceiling of the tide house, take a spool of 

 cord and pass one end of the cord through the fixed pulleys down 

 through the movable pulley attached to the weight, and then secure 

 the end to a screw eye in the ceiling. Next unwind enough addi- 

 tional cord to reach to the tension drum on the gage and cut from the 

 spool. Pass the end of the cord through the small hole provided for 

 the purpose and secure by knotting. The cord is now ready to wind 

 upon the drum as soon as the record paper is installed. 



96. Starting the gage. — The gage is now ready for the installation 

 of the paper (pars. 115-116), adjustment of pencils (pars. 118-119) 

 and entry of comparative note (pars. 120-126) , these processes being 

 described in detail under "Operation of tide station." Both springs 

 on each clock should be wound. If the care of the ga^e is to be turned 

 over to a new observer, he should be given such preliminary instruc- 

 tions as will aid him in interpreting the printed instructions in this 

 volume. 



TIDAL BENCH MARKS 



97. An essential part of the establishment of a tide station is the 

 installation of a system of bench marks to which the observed tides 

 may be ultimately referred. A bench mark may be defined as a defi- 

 nite point on a more or less permanent object used as a reference for 

 elevations. Bench marks established' in the vicinity of a tide station 

 for the purpose of preserving tidal planes determined from the ob- 

 servations are known as "tidal bench marks." These bench marks 

 serve as the basis for elevations which are carried by levels to numer- 

 ous other bench marks established in various parts of the country. 



98. Qualities. — The two principal qualities desired in bench marks 

 are permanency and certainty in identification, . and these qualities 

 should be kept in mind when establishing new bench marks. In a 

 settled community substantial buildings afford excellent locations for 

 the establishment of bench marks. In an undeveloped country a 

 ledge of rocks or a mass of concrete partly buried will serve as a 

 suitable foundation. Except for temporary use only, bench marks 

 should not be located on hydrants, curbstones, trees, or any structure 

 especially liable to destruction or change in elevation. 



99. Standard disks. — Certainty in identification can be best ob- 

 tained by the standard Coast and Geodetic Survey disk bench marks 

 (fig. 18) and these should always be used when possible. The disk, 

 which is made of copper alloy, is 3i/^ inches in diameter and contains 

 the inscription "U. S. Coast and Geodetic Survey Bench Mark" 

 together with other information. It contains a shank for cementing 



