TABLE 899.— TIDES, SEA LEVEL, LEVEL NET 779 



(Nat. Res. Council Bull. 78, 1931.) 



Spring tides. — When moon (new or full) is in line with sun (large tide). 



Neap tide. — When moon is in quadrature with sun (small tide). 



Generally two high and two low each day. Variation in heights of two high and two 

 low = "diurnal inequality." 



River-type tide, steep short-period graph for flood, more inclined and longer for ebb. 

 Extreme case = "bore," tide rises so rapidly it assumes form of wall several feet high. 

 Most famous bores, Tsientang Kiang, China ; Turnagain Arm, Alaska ; Severn and the 

 Wye, England ; Seine in France ; Hoogly, India ; Petitcodiac, Canada. 



Mean sea level (geodetic). — The equipotential surface which the oceans woud as- 

 sume if undisturbed by the tides and effects of wind and weather. Starting with mean sea 

 level at any given initial point the geodesist can determine by precise spirit leveling, the 

 equipotential surface. 



Mean sea level (geographic). — Determined by averaging actual tidal heights over a 

 sufficient period. It is a local or geographic value. It is much disturbed by prevalent winds 

 and local contours. Note difference between average of hourly readings (mean sea level) 

 and half-tide point (because of the shape of the tide height as related to time). On Atlantic 

 coast i tide level lies below mean by about 1/10 ft: on Pacific above by 1/20 ft. Mean 

 tide near rivers varies with rainfall. Nineteen years' observation used for full tide cycle. 

 A fundamental level net has been connected with mean sea level at Portland, Me., via 

 Boston, Mass., Ft. Hamilton, N. Y., Sandy Hook and Atlantic City, N. J., Old Point Com- 

 fort and Norfolk, Va., Brunswick, Ga., Fernandina, St. Augustine, and Cedar Keys, Fla., 

 Biloxi, Miss., Galveston, Tex., San Diego, San Pedro, San Francisco, Calif., Ft. Stevens, 

 Oreg., and Seattle, Wash. The accuracy of high precision leveling is measured by the 

 correction necessary to close circuits, about 0.00063 foot/mile. Mean sea level difference 

 indicated by special adjustment of leveling network in 1929: Portland, Maine, 9 cm 

 higher than Ft. Hamilton; Vancouver, 2 cm higher than Seattle; Galveston, 27 cm higher 

 than St. Augustine; San Diego, 33 cm higher than Galveston; Fort Stevens, 26 cm higher 

 than San Diego ; Isthmus of Panama, Pacific coast, 20 cm higher than Atlantic ; Death 

 Valley, 280 ft (84.1) below sea level; Mount Whitney, 14,495 ft (4418.1 m) above. 



SMITHSONIAN PHYSICAL TABLES 



