SCT. Xffl. SPRING AND NEAP TIDES. 89 



because she is then nearest to the earth. It is evident 

 that the strong tides must happen twice in a month, 

 since in that time the moon is once new and once full. 



The second phenomenon in the tides is the augmen- 

 tation occurring at the time of the equinoxes when the 

 sun's declination (N. 154) is zero, which happens twice 

 every year. The greatest tides take place when a new 

 or full moon happens near the equinoxes, while the 

 moon is in perigee. The inclination of the moon's orbit 

 to the ecliptic is 5 8' 47"-9; hence in the equinoxes the 

 action of the moon would be increased if her node were 

 to coincide with her perigee ; for it is clear that the ac- 

 tion of the sun and moon on the ocean is most direct 

 and intense when they are in the plane of the equator, 

 and in the same meridian, and when the moon in con- 

 junction or opposition is at her least distance from the 

 earth. The spring tides which happen under all these 

 favorable circumstances must be the greatest possible. 

 The equinoctial gales often raise them to a great height. 

 Besides these remarkable variations, there are others 

 arising from the declination or angular distance of the 

 sun and moon from the plane of the equator, which have 

 a great influence on the ebb and flow of the waters. The 

 sun and moon are continually making the circuit of the 

 heavens at different distances from the plane of the 

 equator, on account of the obliquity of the ecliptic and 

 the inclination of the lunar orbit. The moon takes about 

 twenty-nine days and a half to vary through all her de- 

 clinations, which sometimes extend 28| degrees on each 

 side of the equator, while the sun requires nearly 365| 

 days to accomplish his motion from tropic to tropic 

 through about 23^ degrees ; so that their combined mo- 

 tion causes great irregularities, and at times their at- 

 tractive forces counteract each other's effects to a certain 

 extent ; but on an average the mean monthly range of 

 the moon's declination is nearly the same as the annual 

 range of the declination of the sun : consequently the 

 highest tides take place within the tropics, and the low- 

 est toward the poles. The declination of the moon 

 likewise causes the two tides of the same day to rise to 

 unequal heights ; this diurnal inequality of course van- 

 ishes when the moon is in the equator. 

 H2 



