i 271 ) 



II.— THE TIDES. 



(21t.) As introductory to a General Table of the Tides, we shall give a 

 few passages from M. Malte-Brun, explanatory of the subject; and also the 

 results of the extensive observations and profound researches of Professors 

 Airy and Wheioell and the late Sir John Lubbock. 



The water of the sea yields to the slightest impression; and, although its 

 density and weight combine to retain it in constant equilibrium, it is 

 agitated to a certain depth by rapid and varied motions. These motions 

 may be classed according to the manner in which the particles move, and 

 according to the nature of the agents which cause the motion. 



Three kinds of motion may be distinguished in the sea, considered in 

 •reference to their causes. The Tides are sidereal viotions, because they 

 depend upon the influence of the heavenly bodies. General Currents, and 

 the greater number of Particular Currents, have their causes in the very 

 ■element that is agitated by them; these, then, are motions of the sea itself. 

 The third kind comprehends Atmospheric viotions, produced by the action 

 of the Wields. 



The Tides are regular and periodical oscillations, which the seas 

 undergo from the attraction of the celestial bodies, principally those of 

 the moon and sun. 



(218.) Action of the Moon. — Let us first consider the single action of the 

 moon upon the sea ; supposing that luminary to be in the plane of the 

 Equator. It is evident that, if the moon exerted upon all the particles of 

 the sea an equal attraction, and parallel to the earth's centre of gravity, 

 the entire system of the globe, and of the waters which cover it, would be 

 influenced by a common motion, and their relative equilibrium would not 

 suffer any change. The equilibrium is disturbed only by the difference 

 between the attractions which the moon exerts, and the inequality of their 

 directions. Some parts of the globe are directly attracted by the moon ; 

 others only obliquely. The former are in conjunction with the moon ; and 

 a line drawn from the centre of the two planets would pass through their 

 zenith. The latter are in quadrature with the moon — that is to say, a line 

 drawn from the terrestrial centre to their zenith would make an angle of 

 90° with the line which connects the centres of the two planets. The 

 attracting force acting obliquely is decomposed by the obliquity of its angle 

 of incidence ; thus the parts in conjunction being more strongly attracted 

 than those in quadrature, the weight of their particles is diminished. It 

 is necessary, then, to there being an equilibrium in all parts of the sea, 

 that the waters should rise under the moon, in order that the excess of 

 weight of the particles in quadrature, above those in conjunction, may be 

 compensated by the greater height of the latter. 



The waters, however, rise, not only on the side where the attracting 

 planet is, but also on the opposite side ; because, if the planet attract tJie 

 superior waters more than it attracts the centre of the earth, it also attracts 

 this centre more than it attracts the Inferior waters in the opposite hemi- 



