MANUAL OF TIDE OBSERVATIONS ^3 



and the phase of the tide at the time the staff reading was taken in the 

 next column. Any change in the adjustment of the gage should be 

 explained in the column of "remarks," 



209. If there has been no change in the adjustment of the gage, the 

 differences (A— B) should be approximately equal. If an individual 

 value differs materially from the apparent average of all, it must be 

 rejected and excluded from the computation of the mean. The rejec- 

 tion is indicated by encircling the value in que-stion. The differences 

 are now summed and a mean obtained, the result being carried to 

 two decimal places. To this mean difference there is added the pre- 

 liminary scale setting and also any constant that may be necessary to 

 refer to any datum other than staff zero. When a constant is neces- 

 sary it' is furnished by the office. The algebraic sum of these quanti- 

 ties will give the corrected scale setting to be used in the tabulations 

 of the hourly heights and the high and low waters. 



210. If there has been any change in the adjustment of _the gage, 

 such as would be caused by replacing a broken float wire, the intro- 

 duction of kerosene in the float well, a change in the position of the 

 datum line, etc., the differences will form distinct groups. In such 

 cases separate means and corrected scale settings must be computed 

 for each adjustment of the gage. 



211. Sometimes an extreme high or low tide may move the pencil 

 arm to the extreme limit of its motion so that it becomes disengaged 

 from the threaded portion of the pencil screw. If the change in the 

 height of the tide beyond this limit is small, the pencil arm will auto- 

 matically reengage the screw thread after the tide reverses without 

 any change in the adjustment of the gage, but if the tide continues 

 to rise or fall after the disengagement of the pencil arm by an amount 

 equal to or greater than the circumference of the float-wire drum, 

 the adjustment of the gage will be automatically changed. Each com- 

 plete turn of the drum at such times will be found registered by a jog 

 in the record near the edge of the paper, and each such turn will sig- 

 nify a change in the adjustment equal to the circumference of the 

 float drum in use. If such change takes place at the time of high 

 water, the curve will be lowered and the scale readings diminished, but 

 if it takes place at the time of low water, the curve will be raised and 

 the scale readings increased. 



212. In using form 455 the tide roll should be taken as the unit, 

 regardless of the beginning or end of the calendar month, and each 

 comparative note made by the observer on the tide roll should be 

 included in the form. 



HIGH AND LOW WATER TABULATION 



213. Form 138 (figs. 24, 25) is designed for the tabulation of high 

 and low waters, but for a very short series the high and low waters 

 may be tabulated directly in form 248 for the comparison of simulta- 

 neous observations (fig. 27). When form 138 is used the tabulations 

 will in general be arranged by calendar months, but if the entire series 

 of observations does not exceed 31 days the tabulations may be in- 

 cluded on a single sheet of the form. Tide observers who tabulate 

 their own records should, after completing the tabulations for any 

 calendar month, retain a memorandum of any high and low waters at 



