324 OF TIDES. 



calculate that the mean depth of the channel is about twenty. eight 

 fathoms, independently of the magnitude of the resistances of va- 

 rious kinds to be overcome, which require us to suppose the depth 

 from thirty to forty fathoms. In the g^eat river of Amazons, the 

 effects of the tides are still sensible at the streights of Pauxis, 500 

 miles from the sea, after an interval of several days spent in their 

 passage up : for the slower progressive motion of the water no 

 more impedes the progress of a wave against the stream, than the 

 velocity of the wind prevents the transmission of sound in a con. 

 tiarv direction. 



. 



Such are the general outlines of the lunar tides; they are, how- 

 ever, liable to a great variety of modifications, besides their com- 

 bination with the tides produced by the sun. When the moon is 

 exactly over the equator, the highest part of the remoter, or infe- 

 rior, as well as of the nearer or superior tides, passes also over the 

 equator, and the effect of the tide in various latitudes decreases 

 gradually from the equator fo the pole, where it vanishes; but 

 when the moon has north or-south declination, the two opposite 

 summits of the spheroid describe parallels of latitude, remaining 

 always diametrically to each other. Hence the two successive tides 

 anust be unequal at every place except the equator, the greater 

 tide happening when the nearer elevation passes its meridian : and 

 the mean between both is somewhat smaller than the equal tides 

 which happen when the moon passes the equator. This inequality 

 is, however, much less considerable than it would be if the sea 

 assumed at once the form of the spheroid of equilibrium ; and the 

 most probable reasons for this circumstance, are, first, that our 

 tides are partly derived from the equatorial seas ; secondly, that 

 the effects of a preceding tide are in some measure continued so as 

 to influence the height of a succeeding one; and, thirdly, that the 

 tides of a narrow sea are less affected by its latitude than those of 

 3 wide ocean. The height of the sea at low water is the same 

 whatever the moon's declination may be. There is also a slight 

 difference in the tides, according to the place of the moon's nodes, 

 which allows her declination to be greater or less, and this differ- 

 ence is most observable in high latitudes, for instance, in Iceland ; 

 since, in the neighbourhood of the poles, the tide depends almost 

 entirely ou the declination. 



