MANUAL OF TIDE OBSERVATIONS 61 



agree with the hours represented by the cross lines. With the scale 

 in this position, each division is narked on the marigram by a dot. 

 Fourth, these hour dots are referred to the tide curve by lines dravrn 

 through the dots and perpendicular to the datum line. These hour 

 lines are numbered in the same manner as the hour marks automatically 

 made by the time clock. 



203. Checking height datum. — In the portable gage record, the 

 heights are indicated by the horizontal ruling of the profile paper, the 

 zero of which is assumed to correspond to the tide staff zero unless 

 otherwise stated. The height as indicated by the horizontal ruling 

 is to be compared with the actual staff reading as recorded in the 

 comparative notes and allowance made for any difference when tabu- 

 lating the record. The portable gage is designed to be reset whenever 

 necessary to keep the scale and staff readings in agreement. 



204. The determination of the height datum for the standard gage 

 record is radically different from the method used for the portable 

 gage. In the standard gage, no attempt is made to establish in 

 advance any particular relation between gage and staff datum. The 

 aim is to keep the gage datum uniform throughout the entire month, 

 and then, after the record has been removed from the gage, to deter- 

 mine the relation of its datum to the staff zero by the average of the 

 daily comparisons covering the entire month. A special form is 

 provided for the computation and its use is described in the following 

 paragraphs. 



205. Comparative readings (form d'SS). — This form (fig. 23) is 

 used to obtain the relation between the datum line of a standard 

 automatic tide-gage record and the datum adopted for the tabula- 

 tions. The latter datum is .either the zero of the tide staff or tape 

 gage in actual use, or has a definite relation to the same. At primary 

 tide stations it is the aim to maintain a fixed datum for the tabula- 

 tions throughout the entire series of observations, and constants are 

 introduced to take account of an}^ changes in the elevation of the tide 

 staff. The corrected setting as calculated in form 455 represents the 

 scale reading of the datum line as referred to the datum adopted for 

 the tabulations. A movable scale, usually of glass, is used in making 

 these tabulations. In using this scale the side on which the gradua- 

 tions are cut should always be kept dowm next to the paper to avoid 

 errors due to parallax in reading. The numbering of the divisions 

 may be written on the upper surface. 



206. In the first three columns of form 455, the tabulator notes, 

 respectively, the date, the time of staff reading, and the water level 

 as read on the staff or tape, these items being taken directly from the 

 observers notes on the tide roll. The staff reading entered in the 

 form is the mean between the highest and lowest readings recorded, 

 but if a glass tube is used on the staff, the reading entered in the form 

 should be the mean of the tube readings. If a tape gage is used, the 

 word "staff" at the head of the third column may be changed to 

 "tape" and the reading entered should be the mean of the highest and 

 lowest tape readings. 



207. The preliminary scale setting of datum line, to be entered in 

 the heading of the form, may be arbitrarily chosen at any convenient 

 value. This preliminary setting should preferably be of such a value 



735445 0-47-5 



