PHYSICAL ASTRONOMY. 33? 



343. Great extent is necessary, in order that 

 the Sea should be sensibly affected by the ac- 

 tion of the Sun and Moon ; for it is only by the 

 inequality of that action, on different parts of 

 the mass of waters, that their equilibrium is dis- 

 turbed y and such inequality cannot take place, 

 unless a great extent of surface be included. 



The value of cos a z, in the preceding formulas, be- 

 longing to different parts of the same sea, must be 

 considerably different, in order that an oscillation 

 of the waters may be produced. The same is true 

 of the horizontal force, of which sin 2 z is the 

 multiplier. This last is at its maximum at the dis- 

 tance of 45 C from the point where the attracting 

 body is in the zenith. 



344. The tides which are experienced in nar- 

 row seas, and on shores far from the main body 

 of the ocean, are not produced in those seas by 

 the direct action of the luminaries, but are 

 waves propagated from the great diurnal undu- 

 lation, and moving with much less velocity. 



Of this, the tides in the German Sea, and on the 

 coasts of Britain, are remarkable examples. 



The high-water transmitted from the tide in the At- 

 lantic, reaches Ushant between three and four 

 hours after the Moon has passed the meridian, and 

 its ridge stretches N W, so as to fall a little south of 



VOL. II. Y the 



