272 Tj3E tides. 



sphere. These waters will then approach less toward the attracting planet, 

 than the centre of the earth approaches to it. They will remain as far off, 

 from and behind the centre, as the superior waters advance from it on the 

 side of the moon. 



Two promontories, or eminences of water, will therefore be formed by 

 the action of the moon upon the earth ; one on the side toward the moon; 

 the other on the side opposite to it; which gives the sea an appearance of 

 an elongated spheroid, whose great axis will pass through the centre of 

 the moon and of the earth. It is High Tide under the moon and in the 

 opposite point at 180° of distance ; consequently, in the two intermediate 

 points, or at 90° of distance from the moon, the tide will be Loiu. 



The earth, by its rotatory motion, successively presents to the moon, 

 in the space of twenty-four hours, all its meridians, which, consequently, 

 are found by turns, and at an interval of six hours, sometimes under the 

 moon, and sometimes at a distance of 90° from it ; hence it follows that, 

 during the time which passes between the departure of the moon from one 

 meridian, and its return to the same meridian, that is, in the space of a 

 lunar day (which exceeds the solar day by about 50| minutes) , the waters 

 of the sea will ebb twice, and flow twice, in every part of the earth, 

 although in a manner almost insensible in those places which are distant 

 from the path or orbit of the moon. 



(219.) Action of the Sun. — If we now imagine the sun to be in the plane 

 of the Equator, it is evident that, as its action is similar to that ot the 

 moon, it should excite in the ocean an agitation similar to the lunar Tides. 

 Thus the sea would ebb twice, and flow twice, during a solar day; but, on 

 account of the immense distance from the sun, those solar Tides will be 

 much smaller than those which result from the action of the moon. 



On account of the inequality which exists between the solar and lunar 

 days, the action of the sun will sometimes change the position of the lunar 

 Tides, and at other times will unite its influence with that of the moon. 

 In the syzigies, or conjunctions, the action of the moon concurs vdth that 

 of the sun to raise the waters. This is the reason why the highest Tides 

 happen at new and full moon ; or when the moon is in its first and third 

 quarters. In the quadratures, the waters of the sea are depressed by the 

 action of the sun, at the same point where the action of the moon raises 

 them, and reciprocally. Thus the Tides of the quadratures ought to be 

 less. 



(220.) The Height of the Tidal Wave produced by the moon is to that 

 produced by the sun as 100 to 38. When combined, of course, they pro- 

 duce the spring tide ; opposed, they make neaps. The range of these tides 

 is as 138 to 62, or nearly as 7 to 3 ; Newton (from the Severn tides) 

 made it 4-48 to 1, which is far too large. Laplace (from the Brest obser- 

 vations) made it 290 to 1 ; and the late Sir John Lubbock and Dr. 

 Whewell, about 266 to 1. Of course, these relations are very much con- 

 trolled in action by the configuration of the coast or channel. 



(221.) What we have already explained regards the position of the sun 

 and moon on the Equator. Let us now consider these heavenly bodies in 

 their various declinations, and we shall see the elevation vary in the 

 inverse ratio of the cube of the distance of ine water. 



