10 U. S. COAST AND GEODETIC SURVEY 
Hampton Roads 
Pensacola 
San Francisco 
Fic. 2 —Tide curves, illustrating the three types of tide. 
curve for San Francisco illustrates one form of the mixed type of tide. Two high and 
two low waters are seen to have occurred each day, but the forenoon tides differ con- 
siderably from the afternoon tides. 
The distinction between the semidaily and mixed types of tide is based on the fact 
that in the former type morning and afternoon tides resemble each other, while in the 
latter type they exhibit differences. This difference between the two high or low 
waters of a day is known as diurnal inequality and is an important feature of the tide 
in connection with tidal datum planes. It will, therefore, be of advantage to consider 
this feature briefly before taking up the detailed discussion of the different types of tide. 
