153 HYDROGRAPHIC MANUAL Page 38 



The predicted tide curve furnished by the Washington Office is on a long roll of 

 paper 6 inches in width. If it is to be used extensively it will be convenient to construct 

 a small wooden box containing two spools, each of which can be turned by a crank. 

 By these cranks the roll of paper can be wound from one spool to the other, passing 

 over the flat top surface of the box where it is exposed to view between the two spools. 



a. Time correction. — If the tides in the area sounded occur earlier or later than 

 at the reference station, the original numbering of the hour marks must be decreased 

 or increased, respectively, by a corresponding amount. The hour marks on the tide 

 curve are usually not numbered, but the beginning of each day can be identified by 

 a double downward jog in the datum line. With a time allowance of +1 hour this 

 double jog will be taken as 1:00 to adapt the curve to the working ground, but if the 

 time allowance is —1 hour the double jog will be taken as 23:00 of the preceding day. 



6. Range factor. — The allowance for the difference in range can be most conven- 

 iently made by the use of an improvised scale, on a strip of celluloid or paper, repre- 

 senting the product of the original marigram scale by the reciprocal of the ratio oj 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 will be 1/30X 10/8= 1/24. The datum 

 line of the marigram represents the mean sea level, which on the outer coast may be 

 taken as practically the same as half-tide level. To obtain the heights of the tide at 

 a given place, the improvised scale is placed on the marigram with its zero at a dis- 

 tance 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 relation of the plane of reference 

 to half-tide level can usually be taken directly from the bench-mark data. 



Wliere previous tide observations are not available, an approximate setting of the 

 improvised scale on the datum line can be obtained by multiplying the ratio of the 

 ranges for the workmg ground by the depression of the datum plane below mean sea 

 level at the reference station. The depression can be foimd in the List of Reference 

 Stations preceding 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 Listof 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. 



153. Magnetic Observations at Sea 



Magnetic observations at sea with special instruments will ordinarily be taken 

 on a nonmagnetic vessel or on a survey vessel by an observer especially tramed for 

 this work. When such observations are desired, special instructions will be issued for 

 them. 



Recent experiments have disclosed that, under the best conditions, a ship swing 

 may be made in a nonmagnetic small boat which will give a reasonably accurate value 

 of the magnetic variation. If practicable, the swing should be made where there is 

 a depth of water of 50 fathoms or more so that the compass is not likely to be influ- 

 enced by any local magnetic attraction in the ocean bottom. 



The boat selected for this purpose should be as free as possible from magnetic 

 material. Bronze fittings should be substituted for iron fittings where practicable, 



