138 Examination of St. Pierre's Hypothejis 



fun is moft diftant from the equator. When the moon's 

 in the equator, the tides are equally high in both parts of the 

 lunar day ; but as the moon declines towards either pole, the 

 tides are alternately higher and lower at places having north 

 •or fouth latitude : while the fun is in the northern figns, the 

 p-reater of the two diurnal tides in our climates is that arifing 

 from the moon above the horizon : when the fun is in the 

 fouthern figns, the greateft is that arifing from the moon 

 below the horizon. Thus the eveniug tides in fummer are 

 obferved to exceed the morning tides, and in winter the 

 niominsr tides exceed the evening tides : the difference at 

 Briftol is found to be 15 inches, at Plymouth 12. It would 

 be (till greater, but that a fluid always retains an impreffed 

 motion for fome time, and confequently the preceding tides 

 always 3Be<5t thofe that follow. 



If the earth were covered all over with the Tea to a great 

 depth, the tides would be regularly fubfervrent to thefe laws; 

 but various caufes combine to produce a great diverfity of 

 effect, according to the peculiar fituation and circum fiances 

 of places, fhoals, fords, and ftraits : thus, a flow and imper- 

 ceptible motion of a large body of water, fuppofe two miles 

 deep, will be fufiicicnt to elevate its furface ten or twelve feet 

 in a tide's time; whereas, if the fame quantity of water is 

 forced through a narrow channel forty or fifty fathoms deep, 

 it produces a vcrv rapid ftream, and of courfe the tide is found 

 to fet ftrongeft in thofe places where the fea grows narroweft, 

 the fame quantity of water beino; conftrained to pafs through 

 a fmaller pafiage, as in the ftraits between Portland and Cape 

 la Hocme in Normandy ; and it would be ftill more fo~ be- 

 tween Dover and Calais, if the tide coming round the ifland 

 did not cheek it. 



The fhoalnefs of the fea and the intercurrent continents 

 are the reafons why the tides in the open ocean rife but to 

 verv inconfiderable heights, when compared to what they do 

 in wide-mouthed rivers opening in the direction of the ftream 

 of the tide ; and that high water is fome hours after the 

 moon's appulfe to the meridian, as it is obferved upon all 

 the weftern coaft of Europe and Africa from Ireland to the 

 Cape of Good Hope j in all which a fouth-weft moon makes 



fcfiffh 



