TIDES. 



37 



celestial body which influences the rise and fall of the waters of the sea. 

 We have already said that the sun asserts an influence on the waves. 

 It is true that, in consequence of its great distance, this only amounts 

 to a thirty-eight-hundredth part of that of the earth's satellite. The 

 inequality which exists between the solar and lunar days the latter 

 exceeding the first by fifty-four minutes has also the effect of adding 

 to or subtracting from this force alternately. When the sun and 

 moon are in conjunction (Fig. 6), or in opposition, that is to say, 

 placed upon the same right line, their attraction on the sea is com- 

 bined, and a spring tide is produced. This happens at the period of 



Tlie Sun. 



South Pole. 



Fig. 6. Lunar-Solar Tides. 



the syzygies the period of new and full moon. At the period of the 

 quadrature, or the first and last quarters, the solar action, being opposed 

 to that of lunar attraction, tends to produce a sensibly weaker tide. 



These effects are never produced instantaneously ; but, the impulse 

 once given, it will continue to influence the tides for two or three days, 

 the highest and lowest tides being nearly in the proportion of 138 to 63, 

 or of 7 to 3. The highest tides occur at the equinoxes, when the 

 moon is in perigee ; the lowest at the solstices, when it is in apogee. 

 In our. ports, and along the coast, the water rises twice in twenty-four 

 hours, when it is said to be high water ; when it retires, it is low 

 water : they are respectively iheflux and reflux of the waves. 



