2. PLANS AND PREPARATIONS 



41 



made by the use of an improvised scale, on a 

 strip of celluloid or paper, representing the 

 product of the original marigram scale by 

 the reciprocal of the ratio of the ranges, as 

 taken from the Tide Tables. Thus, if the 

 marigram scale is 1/30 and the ratio of the 

 ranges 0.8, then the improvised scale is 

 placed on the marigram with its zero at a 

 distance below the datum line equal to the 

 depression of the plane of reference below 

 mean sea level or half-tide level. For the 

 Atlantic Coast, this depression is approxi- 

 mately the half range of tide at the working 

 ground. For the Pacific Coast, it will be the 

 half range of tide plus the diurnal low-water 

 inequality. Where there have been previous 

 tide observations in the locality, the rela- 

 tion of the plane of reference to half-tide 



level can usually be taken directly from the 

 bench-mark data. 



Where previous tide observations are not 

 available, an approximate setting of the im- 

 provised scale on the datum line can be ob- 

 tained by multiplying the ratio of the ranges 

 for the working ground by the depression of 

 the datum plane below mean sea level at the 

 reference station. The depression can be 

 found in the List of Reference Stations pre- 

 ceding the daily predictions in the Tide 

 Tables. Thus, if the ratio of the ranges at 

 Oakland (the working ground) is 1.2 and 

 the plane of reference at San Francisco (the 

 reference station) is 3.0 feet below mean sea 

 level (from List of Reference Stations), then 

 the setting of the scale on the datum line 

 of the marigram would be 3.0 feet x 1.2 = 

 3.6 feet. 



