MANUAL OF TIDE OBSERVATIONS 



41 



from any obstruction. When a tape gage is used at the station, 

 levels must be run from the reading mark of the gage to the bench 

 marks, the relation of this reading mark to the tape gage datum being 

 determined from the position of the plane of flotation (pars. 82-86). 

 112. Before beginning the leveling care must be taken to have the^ 

 instrument in adjustment and also to see that the bottom of the 

 leveling rod corresponds exactly with the zero of the graduations. 

 The latter is important if some sights are to be taken directly on the 

 tide staff or on a graduated tape. Foresights and backsights should 

 be approximately equal. The leveling between the marks must be 

 checked by a forward and a backward line and the closing error in 

 feet must not exceed 0.035 K^ in which K is the distance in statute 

 miles leveled between adjacent bench marks. For convenience of 

 use the following table is given : 



Distance between bench 

 marks (feet) 



500 or less 



1,000 . 



2,000 



Maximum 

 error allowed 



Foot 

 0.011 

 .015 

 .021 



Distance between bench 

 marks (feet) 



3,000 

 4,000 

 5,000 



Maximum 

 error allowed 



Foot 

 0.027 

 .030 

 .034 



If the difference between the results from the forward and back- 

 ward lilies between any two bench marks exceeds the allowable error, 

 both the forward and backward lines between the marks must be 

 repeated until an acceptable agreement is obtained. No one of the 

 questioned values is to be used with the new levels to obtain the 

 agreement. Form 258, leveling record — ^tide station, is to be used for 

 this leveling. 



OPERATION OF TIDE STATION 



113. The satisfactory functioning of a tide gage depends largely 

 upon the observer in charge, and he should be a person with some 

 knowledge of mechanics and a sufficient scientific training to under- 

 stand the importance of careful observations. 



114. Summary of observer's duties. — Exclusive of work incident 

 to density and temperature observations and the tabulation of 

 records described elsewhere in this volume, the duties of the tide 

 observer at a primary tide station may be summarized as follows, 

 the details of the work being described in separate paragraphs: 



Daily. — Inspect tide station, enter comparative time and staff note 

 (pars. 120-126) on tide roll and in form 660 (par. 136), correct gage 

 clocks if necessary (par. 123), readjust pencils for wearing away of 

 lead (par. 128), wind up tension weight, and enter in form 660 any 

 item of interest relating to the station (par. 136). 



Semiweekly. — ^Wind both clocks. Although these are 8-day clocks, 

 semiweekly winding insures against stoppage that might result from 

 an unexpected interruption to the observer's daily visits. 



Weekly. — Mail form 660 to office after completing information 

 required on back of form. 



Monthly. — Change paper on gage (par. 117), clean pencil screw 

 (par. 131), and forward records to office (pars. 137-139). 



