39 



Pi, principal solar diurnal, whose speed is the difference 

 between those of the solar day and tropical year, 

 15-0°.041,068,6 = 14°. 958,931,4. 

 The solar elliptic diurnal components corresponding to Ji and Qi 

 are too small to be recognized. Daily land and sea beeezes and daily 

 variations in atmospheric pressure may however give rise to small 

 fluctuations having the period of the mean solar day. A meteoro- 

 logical component. Si, with a speed of 15° per hour is therefore 

 recognized. 



71. Long period components. — The following long period compo- 

 nents have already been developed in the discussion of the solar and 

 lunar equilibrium tides: 



The lunar fortnightly (par. 42). — Tliis component is conventionally 

 represented by the symbol Mf . Its speed is twice that of the tropical 

 month, or 1°. 098, 033,0 per mean solar hour. 



The lunar monthly (par. 43). — Conventionally represented as Mm. 

 Its speed is that of the anomalistic month, or 0°. 544, 374, 7 per mean 

 solar hour. 



The solar semiannual (par. 44). — Conventionally represented as Ssa. 

 Its speed is twice that of the tropical year, or 0°. 082, 137,2 per mean 

 solar hour. 



In addition, recurring seasonal meteorological effects produce a 

 solar annual component, designated Sa, whose speed is that of the 

 tropical year, or 0°. 041, 068, 6 per solar hour. 



72. Overtides. — A distortion of the tides is produced in the compara- 

 tively shallow waters of estuaries and other coastal areas. This dis- 

 tortion gives rise to tidal components whose speeds are multiples of 

 the speeds of the astronomical components heretofore developed. 

 They are called overtides because of their analogy to overtones in the 

 theory of musical tones. The only overtides of sufficient magnitude 

 to be of importance are those of the principal lunar and solar com- 

 ponents M2 and S2. They are designated M4, Me, Mg, and S4 and 

 Se, the subscripts denoting the ratio of their speeds to that of the mean 

 lunar or solar day. 



73. Compound tides. — Besides producing overtides, the distortion 

 of the tides in shallow water gives rise to components whose speeds 

 are the sums or differences of the speeds of the elementary components. 

 The recognized compound tides are MS, with a speed of m2+S2; MN 

 with a speed of m2+n2, MK with a speed of m2+ki, 2MK with a 

 speed of m4— ki, and 2 SM with a speed of S4— m^. These components 

 are generally quite small. Certain other compound tides have the 

 same speed as some of the primary components and therefore unite 

 with them. 



74. Tide depending on the fourth power of the moon's parallax. — It 

 has been shown in paragraph 32 that the lunar equilibrium tide due to 



