VOL. XIX.] PHILOSOPHICAL TRANSACTIONS. 147 



nearest either to the zenith or nadir of the place : whence it is that the moon 

 in the northern signs, in this part of the world, makes the greatest tides when 

 above the earth, and in southern signs, when under the earth ; the effect being 

 always the greatest where the moon is farthest from the horizon, either above 

 or below it. And this alternate increase and decrease of the tides has been ob- 

 served to hold true on the coast of England, at Bristol by Capt. Sturmy, and 

 at Plymouth by Mr. Colepresse. 



But the motions hitherto mentioned are somewhat altered by the libration of 

 the water, by which, though the action of the luminaries should cease, the 

 flux and reflux of the sea would for some time continue: this conservation of 

 the impressed motion diminishes the differences that otherwise would be be- 

 tween two consequent tides, and is the reason why the highest spring tides are 

 not precisely on the new and full moons, nor the neaps on the quarters ; but 

 generally they are the third tides after them, and sometimes later. 



All these things would regularly happen, if the whole earth were covered 

 with sea very deep ; but by reason of the shoalness of some places, and the 

 narrowness of the straits, by which the tides are in many cases propagated, 

 there arises a great diversity in the effect, and not to be accounted for, without 

 an exact knowledge of all the circamstances of the places ; as of the position 

 of the land, and the breadth and depth of the channels by which the tide flows: 

 for a very slow and imperceptible motion of the whole body of the water, where 

 it is, for instance, 2 miles deep, will suffice to raise its surface 10 or 12 feet in 

 a tide's time ; whereas, if the same quantity of water were to be conveyed on 

 a channel of 40 fathoms deep, it would require a very great stream to effect it, 

 in so large inlets as are the channel of England and the German ocean ; whence 

 the tide is found to set strongest in those places where the sea grows narrowest; 

 the same quantity of water being to pass through a smaller passage : this is 

 most evident in the straits between Portland and Cape de Hogue in Nor- 

 mandy, where the tide runs like a sluice ; and would be yet more between 

 Dover and Calais, if the tide coming about the island from the north did 

 not check it. And this force, being once impressed on the water, continues 

 to carry it above the level of the ordinary height in the ocean, particularly 

 where the water meets a direct obstacle, as it is at St. Malo's ; and where it 

 enters into a long channel, which running far into the land, grows very strait 

 at its extremity ; as it is in the Severn Sea, at Chepstow and Bristol. 



From this shoalness of the sea, and the intercurrent continents, it is, that 

 in the open ocean the time of high-water is not at the moon's appulse to the 

 meridian, but always some hours after it ; as it is observed on all the west coast 

 of Europe and Africa, from Ireland to the Cape of Good-Hope: in all which a 



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