TIDES. 49 



of years, produces a corresponding effect upon the tides; and it 

 is in this way that the differences which we find in morning 

 and evening tides, in ordinary and equinoctial tides, and in 

 summer and winter tides, are accounted for. 



Diurnal and Semi-diurnal Tides. The difference in the range 

 of diurnal and semi-diurnal tides, which is very apparent in 

 many localities, is attributable to variation in the attractive 

 power of the sun and moon on opposite sides of the earth, 

 resulting from declination. 



Speaking generally, during one half of the year the highest 

 tides occur in the morning, and during the other half of the 

 year in the evening. This difference, in some cases, amounts to 

 several feet. Indeed, in the Gulf of Mexico, and in some positions 

 near Behring Strait, the difference of morning and evening tides 

 is so great that, in certain parts of the lunation, there appears 

 to be only one tide in the day. 



At Singapore this inequality amounts to as much as 6 feet, 

 and around the coast of Australia it is also very marked. 



This difference between morning and evening tides is, how- 

 ever, greatly affected by local circumstances, and seems to follow 

 no general law. 



Equinoctial Spring Tides. When the sun is vertical over the 

 equator, as is the case at the vernal and autumnal equinoxes 

 in March and September, its attractive power upon the water 

 immediately beneath it is then greatest; and when the lunar 

 tides, at new and full moon, coincide with the solar tides about 

 this time, which, owing to the small declination of the moon, 

 they do more closely than at other times, abnormally high and 

 low tides occur. These are known as " equinoctial sprino- 

 tides." 



Difference in Range of Summer and Winter Tides. The earth, 

 revolving in her elliptical orbit, is nearer to the sun at mid- 

 winter, and further away from it at mid-summer than at any 

 other periods of the year. Hence the attractive power which 

 the sun exercises upon the earth is greater in winter than in 

 summer, and the range of the tides is correspondingly affected, 

 that of the winter tides being increased, while that of the 

 summer tides is diminished. The declination of the sun, which 

 reaches its maximum on the 21st of June and 21st of December 

 respectively, however, considerably modifies the sun's influence 

 on the tides at these periods; and it is in consequence of this 



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