TIDAL DATUM PLANES . D7 
height of the tide at regular intervals. For certain purposes the printing gages are 
preferred, but for general purposes the curve-tracing gages have several advantages, 
among which may be mentioned the visualizing of any breaks in the record, whether due 
to stoppage or accidental change in adjustment. The curve-tracing gage furthermore 
permits the recording and studying of rapid changes in level and of any unusual features 
in the rise and fall of the tide. 
The Coast and Geodetic Survey makes use of two types of. curve-tracing tide 
gages, known respectively as the standard gage and the portable gage. The standard 
gage is generally used for tide observations except for short series or where local con- 
ditions are not suitable, in which cases the portable gage is used. 
Standard Tide Gage 
The essential parts of the standard tide gage consist of a clock that moves a roll 
of paper forward at a uniform rate and a float that is free to rise and fall with the tide 
and which is so connected with a pencil that the latter moves perpendicularly to the 
motion of the paper and proportional to the rise and fall of the tide. The combined 
motion of paper and pencil produces a continuous curve known as the tide curve, which 
shows the rise and fall of the tide to a reduced scale. From this tide curve the height 
of the tide at any given instant during the period of observations may be determined. 
In addition to the clock which moves the paper, or motor clock, there is another 
clock, known as the time clock, the function of which is to make a short horizontal 
mark on the record at the beginning of each hour. The paper used on the gage is 
about 13 inches wide and is furnished in rolls about 70 feet long. Since the motor clock 
feeds the paper at the rate of 1 inch per hour, a roll is sufficient for 1 month of record. 
The approximate over-all dimensions of the standard gage are: length, 3 feet; width, 
1% feet; height, 1 foot. A view of the instrument is given in Figure 11. 
Fic. 11.—Standard tide gage. 
