102 THEORY OF THE TIDES. [SECT. xm. 



SECTION XIII. 



Tides Forces that produce them Three kinds of Oscillations in the Ocean 

 The Semidiurnal Tides Equinoctial Tides Effects of the Declination 

 of the Sun and Moon Theory insufficient without Observation Direction 

 of the Tidal Wave Height of Tides Mass of Moon obtained from her 

 Action on the Tides Interference of Undulations Impossibility of a 

 Universal Inundation Currents. 



ONE of the most immediate and remarkable effects of a gravi- 

 tating force external to the earth is the alternate rise and 

 fall of the surface of the sea twice in the course of a lunar 

 day, or 24 h 50 m 28 s of mean solar time. As it depends upon 

 the action of the sun and moon, it is classed among astro- 

 nomical problems, of which it is by far the most difficult and 

 its explanation the least satisfactory. The form of the sur- 

 face of the ocean in equilibrio, when revolving with the earth 

 round its axis, is an ellipsoid flattened at the poles ; but the 

 action of the sun and moon, especially of the moon, disturbs 

 the equilibrium of the ocean. If the moon attracted the 

 centre of gravity of the earth and all its particles with equal 

 and parallel forces, the whole system of the earth and the 

 waters that cover it would yield to these forces with a com- 

 mon motion, and the equilibrium of the seas would remain 

 undisturbed. The difference of the forces and the inequality 

 of their directions alone disturb the equilibrium. 



It is proved by daily experience, as well as by strict mathe- 

 matical reasoning, that, if a number of waves or oscillations 

 be excited in a fluid by different forces, each pursues its 

 course and has its effect independently of the rest. Now in 

 the tides there are three kinds of oscillations depending on 

 different causes, and producing their effects independently 

 of each other, which may therefore be estimated separately. 



