TIDAL DATUM PLANES 29 
Fic, 12.—Portable tide gage. 
are necessary since in tabulating tide records it is customary to tabulate times to the 
nearest tenth of an hour and heights to the nearest tenth of a foot, except in regions of 
little range of tide, in which case heights are tabulated to the nearest half tenth or 
even closer. 
The standard gage does not employ cross-section paper. ‘The record is made on a 
roll of plain paper on which the time scale is 1 inch to the hour, each roll generally 
containing the record for a calendar month. At the beginning of each hour the hour- 
marking device on the gage causes the pencil to make a short horizontal stroke, thus 
