SOLAR AND LUNAK TIDES. 



400 times greater than that of the sun, if their entire attractions 

 on the earth were equal, it will be less than this, in the ratio of 

 177| to 1, inasmuch as the entire amount of the moon's attraction 

 is less than that of the sun in that proportion. The moon's 

 power to raise a tide is, therefore, greater than that of the sun in 

 the ratio of 400 to 177^, or of 2^ to 1. Other calculations make it 

 about 2^ to 1. 



7. It appears, therefore, that there is a solar as well as a lunar 

 tide ; and as the lunar tidal wave follows the diurnal motion of 

 the moon, the solar tidal wave follows that of the sun. When 

 the sun and moon are, therefore, either on the same or on 

 opposite sides of the earth, which they are at the epochs of new 

 and full moon, the two tidal waves will be superposed; but 

 when their directions are most removed one from the other, 

 which they are when the moon is in the quarters, the two tidal 

 waves are most separated, being also ninety degrees of the 

 -earth's surface apart. 



In the one case, a tide is produced corresponding to the sum 

 of the effects of the actions of the moon and sun ; and, in the 

 other case, to their difference. 



8. These circumstances will be better understood by referring 

 to the illustrative diagrams. In ng. 2, s represents the sun, M the 

 moon when new, and 3i' when full. The solar and lunar tidal 

 waves in these cases coincide, and are heaped one upon the other, 

 producing what are called SPRING TIDES. 



In fig. 3, s represents the sun, and M and M' the moon in 

 the quarters. In this case, the solar tidal wave is ninety degrees 

 or a quarter of the earth's circumference from the lunar tidal 

 wave, and the waters which form it are necessarily drawn from 

 the side of the earth on which the lunar tide places itself. It 

 is evident, therefore, that, in this case, the solar tide being 

 formed at the expense of the lunar, the latter will be much less 

 high. The tides in this case are called NEAP TIDES. 



9. If physical effects followed immediately, without any appre- 

 ciable interval of time, the operation of their causes, then the 

 tidal wave produced by the moon would be on the meridian of 

 the earth directly under and opposite to that luminary; and 

 the same would be true of the solar tides. But the waters of the 

 globe have, in common with all other matter, the property of 

 inertia, and it takes a certain interval of time to impress upon 

 them a certain change of position. Hence it follows that the 

 tidal wave produced by the moon is not formed immediately 

 under that body, but follows it at a certain distance. In conse- 

 quence of this, the tide raised by the moon does not take place 

 for two or three hours after the moon passes the meridian ; and 



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