MANUAL OF TIDE OBSERVATIONS 



79 



above or below the zero of the scale, according to whether the value 

 from the formula is negative or positive. 



252. The graph is laid over the marigram with the red horizontal 

 line in coincidence with the datum line if a standard gage record, or 

 with the scale zero of the portable gage record (assuming that this 

 corresponds to the tide staff zero), and the hour marks of the graphic 

 scale and of the marigram in coincidence. In case a time allowance 

 is to be made, the graph is shifted to the right or to the left, according 

 to the amount of time allowance. 



253. The spaces between the horizontal lines are numbered . with 

 figures somewhat larger than those used for the scale lines, beginning 



-7 

 -6' 



-S 



-3.5 

 "J - 



Ret f/ine /< ' he ph reed or e/ahirr 





+ or 



/?< T^/ffre 1 o be pi vrceafo i zefv 



ie of'i fcrndc faf qo ugfe rt 'caret 



+ / 



+^ 



- /lie of f 'ot^eti >/e grari lye re< ore/ 



FiGUEB 28. — Graph for obtaining tide reducers directly from marigram. 



with "0" for the space just above the zero (0) line and numbering 

 consecutively above and below this space, using the plus ( + ) sign 

 before the numerals for the lower spaces. These numbers will be 

 the tide reducers to integral feet for all portions of the tide curve 

 falling within the space so marked. 



254. An example of a graphic scale is given in figure 28. For the 

 standard-gage record, assuming that "a?," the height of the plane of 

 reference above the zero of the tide staff, is 2.3 feet, and that "y," 

 the height of the datum line above the zero of tide staff, is 5.1 feet, 

 the formula x—y — 0.1 gives —3.5 feet as the scale reading at which 

 the red line is drawn, this line to be placed in coincidence with the 

 datum line of the marigram when the scale is in use. For the porta- 

 ble gage assuming that "a?," the height of plane of reference on tide 

 staff j is 2.9 feet, and taking "y" as zero, the formula gives +2.2 feet 

 as the scale reading for the red line, which is to be placed in coinci- 

 dence with the zero of the marigram scale when in use. 



