FLUCTrATIONS OF THE TIDES. 99 



course around the globe ; owing to the incessant mobility of the 

 waters, they mix and are confused, and it is by calculation alone that 

 we can discriminate in their common mass the part that is to be re- 

 ferred to each of the two heavenly bodies. These two united in- 

 tumescences move together around the earth in a direction from east 

 to west ; that is to say, in the opposite direction to the rotation of 

 the globe. Serving thus as a drag upon the planet, they must in 

 the long run lead to that slackening of its speed, which the calcula- 

 tions and deductions of Meyer, Tyndall, Joule, Adams, and Delaunay 

 lead us to consider as inevitable.* 



"When the moon, called new, turns its dark face towards us, and is 

 thus in nearly the same direction as the sun relatively to the earth, 

 the attractions of the two great celestial bodies join together, and the 

 two tidal waves, raised at the same time towards the same point of 

 space, are exactly superposed. They form those tides of syzygy or 

 high water, called spring-tides, which rise to such great heights along 

 our shores. At the time of full moon^ that is to say, when the 

 satellite, entirely lighted, is in direct opposition to the sun, new tides 

 of syzygy not less elevated than the first' are formed; for under the 

 influence of the heavenly bodies situated opposite to each other a 

 double intumescence is simultaneously produced on both sides of the 

 earth. During none of the. other phases of the moon does this coin- 

 cidence exist ; at the time of quadrature, the two great movements 

 of the waves oppose one another, and the tidal wave, which represents 

 then the lunar wave diminished by the entire solar wave, is less 

 elevated than during the other phases of the moon. If the two 

 attracting forces were equal in power, the neutralization of the tide 

 would be complete, and the level of the sea would remain undis- 

 turbed. 



To give an idea of the fluctuations which occur during the course 

 of an entire tide, under the influence of the heavenly bodies, and 

 which are variously modified by the atmospheric currents, the form 

 of the coast, and inequalities of the bed of the sea, we borrow the 

 following figure from Beardmore. 



The periods of the tides are exactly those of the bodies which raise 

 them. The semi-diurnal period of 12 hours 25 minutes is comprised 

 between the passage of the moon over the two opposite meridians 

 of the earth. The diurnal period, during which the ocean swells 

 and subsides twice, corresponds exactly to the duration of one ap- 

 parent rotation of the satellite around our planet. There is the 

 * See in Vol. I. tlie section entitled, The Earth in Space, 

 H 2 



