574 Tables 721 and 722 



TABLE 721. — Tides, Sea-Level, Level Net 

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



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



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



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 would assume 

 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 \ 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 Comfort 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 differences : 



Portland 16.94 cm higher than Ft. Hamilton. 

 Vancouver 10.28 cm higher than Seattle. 

 Galveston 24 cm higher than St. Augustine. 

 San Diego 40 cm higher than Galveston. 

 Fort Stevens 79 cm higher than San Diego. 



Isthmus Panama, Pacific coast 20 cm higher than Atlantic. 



Death Valley is 276 ft. (84.1 m) below sea-level, Mount Whitney 14,496 ft. (4418.4 m) 

 above. 



TABLE 722. — Magnetic and Electric Data for Sun and Earth 

 (Chapman, Cosmical magnetic phenomena, Nature, 124, 19, 1929.) 



Sun's magnetic field too small to be measured by direct effects on earth ; measured by 

 Zeeman effect on spectrum lines. 



Earth's magnetic axis inclined 12° to rotation axis. 



Sun's magnetic axis inclined 4° to rotation axis. 



Polarity of both same relation to direction of rotation. 



Earth's field rotates at same speed as nearly rigid earth. 



Sun's field and magnetic axis rotate more slowly than solar surface (31, 26 days, 

 respectively). 



Earth : Polar intensity of field f gauss. 



Sun : Estimated 50 gauss in reversing layer. Intense local fields frequent, 3000 gauss. 

 The magnetic field of spots reverses each cycle (Proc. Astron. Soc. Pacific, 41, 136, 1929). 

 The polarity of leading spot in a bipolar group in N. hemisphere is opposite that in the 

 S. hemisphere — relationship reverses each new sun-spot cycle •'• complete magnetic cycle 

 is double sun-spot cycle. 



Specific resistances : Earth Sun (Chapman loc. cit.) 



Heaviside layer, io 10 Reversing layer, 3 X T o 10 



Dry earth, io 15 to io ls Photosphere, io 8 , T, 10000 K. 



Sea water, 2 X io 10 Center, 3 X io 3 , T, 4 X io 7 



200-600 m deep, 3X10" 



Drift currents in sun, + ions easterly. 



Further characteristics of spots: (Milne, M. N. 90, 487, 1930; Russell.) Umbra (dark 

 center), 800 (very small) to 80,000 km across: penumbra may reach 240,000 km. Gen- 

 erally short-lived. A few last several (3) rotations, very rarely 6; one in 1840, 18 months. 

 Most occur in 2 belts 5 to 40 N. and S. latitudes, often occur in pairs (see above). 

 Umbra temperature 4000 K. Evershed gives velocity of outburst from spot 2 km/sec. 



Smithsonian Tables 



