difference between the attractive force of the sun and moon.-The waters at Z and N will be more elevated than the waters at D and A, because the moon's attractive force is four* times that of the sun.

THEOREM VI. The spring-tides do not happen exactly on the day of the change or full moon, nor the neap-tides exactly on the days of the quarters, but a day or two afterwards.

When the attractions of the sun and moon have conspired together for a considerable time, the motion impressed on the waters will be retained for some time af, ter their attractive forces cease, and consequently, the tide will continue to rise. In like manner at the quarters, the tide will be the lowest when the moon's attraction has been lessened by the sun's for several days tor gether. If the action of the sun and moon were suddenly to cease, the tides would continue their course for some time, like as the waves of the sea continue to be agitated after a storm.

THEOREM VII. When the moon is nearest to the earth, or in Perigee, the tides increase more than in similar circumstances at other times.

For the power of attraction increases as the square of the distance of the moon from the earth decreases; con

* Sir Isaac Newton, Cor. 3. Prop. XXXVII. Book III. Principia, makes the force of the moon to that of the sun, in raising the waters of the ocean, as 4.4815 to 1; and in Corol 1. of the same proposition, he calculates the height of the solar tide to be 2 feet 0 inches 4, the lunar tide 9 feet 1 inch and by their joint attraction 11 feet 2 inches; when the moon is in Perigee. the joint force of the sun and moon will raise the tides upwards of 134 feet.-Sir Isaac Newton's measures are in French feet in the Principia. I have turned them into English feet.

Mr. Emerson, in his Fluxions, Section III. Prob. 25. calculates the greatest height of the solar tide to be 1 63 feet, the lunar tide 7 28 feet, and by their joint attractions 8.91 feet, making the force of the sun to that of the moon as 1 to 4 4815.

Dr. Horsley, the late Bishop of St Asaph. estimates the force of the moon to that of the sun as 5.0469 to 1. See his edition of the Principia, lib. 3. sect. 3. Prop. XXXVI. and XXXVII.

Mr. Walker, in Lecture 11th of his Familiar Philosophy, states the influence of the sun to be to the influence of the moon to raise the water, as 3 is to 10, and their joint force 13.

sequently, the moon must attract most when she is nearest to the earth.

THEOREM VIII. The spring tides are greater a short time before the vernal equinox, and after the autumnal equinox, viz. about the latter end of March and September, than at any other time of the year. (Plate III. Figure 3.)

Because, the sun and moon will then act upon the equator in the direction of a f B; consequently, the spheroidal figure of the tides will then revolve round its longer axis, and describe a greater circle than at any other time of the year; and, as this great circle is described in the same time that a less circle is described, the waters will be thrown more forcibly against the shores in the former circumstance than in the latter.

THEOREM IX. Lakes are not subject to tides; and small inland seas, such as the Mediterranean and Baltic, are little subject to tides. In very high latitudes, north or south, the tides are also inconsiderable.

The lakes are so small, that when the moon is vertical she attracts every part of them alike. The Mediterranean and Baltic seas have very small elevations, because the inlets by which they communicate with the ocean are so narrow, that they cannot, in so short a time, receive or discharge enough to raise or lower their surfaces sensibly.

THEOREM X. The time of the tides happening in particular places, and likewise their height, may be very different according to the situation of these pla

ces.

For, the motion of the tides is propagated swifter in the open sea, and slower through narrow channels or shallow places; and, being retarded by such impedi ments, the tides cannot rise so high.

General Observations.

The new and full moon spring-tides rise to different heights.

The morning tides differ generally in their rise from the evening tides.

In winter, the morning tides are highest.

In summer, the evening tides are highest.

The tides follow, or flow towards the course of the moon, when they meet with no impediment. Thus, the tide on the coast of Norway flows to the south, (towards the course of the moon) from the North-cape in Norway to the Naze, at the entrance of the Scaggerac, or Cattegate Sea, where it meets with the current which sets constantly out of the Baltic Sea, and consequently, prevents any tide rising in the Scaggerac. The tide proceeds to the southward along the east coast of Great Britain, supplying the ports successively with high water, beginning first on the coast of Scotland. Thus, it is high water at Tynemouth Bar at the time of new and full moon about three hours after the time of high water at Aberdeen; it is high water at Spurn-head about two hours after the time of high water at Tynemouth Bar; in an hour more it runs down the Humber, and makes high water at Kingston upon Hull; it is about three hours running from Spurn-head to Yarmouth Road; one hour in running from Yarmouth Road to Yarmouth Pier; 24 hours in running from Yarmouth Road to Harwich; 1 hour in passing from Harwich to the Nore; from whence it proceeds up the Thames to Gravesend and London. From the Nore, the tide continues to flow to the southward to the Downs and Godwin Sands, between the north and south Foreland in Kent, where it meets the tide which flows out of the English Channel through the Strait of Dover.

While the tide, or high water, is thus gliding to the southward along the eastern coast of Great Britain, it also sets to the southward along the western coasts of Scotland and Ireland: but on account of the obstructions it meets with by the Western Islands of Scotland, and the narrow passage between the north-east of Ireland, and the south-west of Scotland, the tide in the Irish sea comes round by the south of Ireland through St. George's

Channel, and runs in a north-east direction till it meets the tide between Scotland and Ireland, at the north-west part of the Isle of Man. This may be naturally inferred from its being high water at Waterford above 3 hours before it is high water at Dublin, and it is high water at Dundalk Bay and the Isle of Man nearly at the same time. That the tide continues its course southward may be inferred from its being high water at Ushant, opposite to Brest in France, about an hour after the time of high water at Cape Clear, on the southern coast of Ireland. Between the Lizard Point in Cornwall and the island of Ushant, the tide flows eastward, or east north-east, up the English Channel, along the coasts of England and France, and so on through the strait of Dover till it comes to the Godwin Sands or Galloper, where it meets the tide on the eastern coast of England, as has been observed before. The meeting of these two tides contributes greatly towards sending a powerful tide up the river Thames to London; and when the natural course of these two tides has been interrupted by a sudden change of the wind, so as to accelerate the tide which it had before retarded, and to drive back that tide which had before been driven forward by the wind, this cause has been known to produce high water twice in the course of three or four hours. The above account of the British tides seems to contradict the general theory of the motions of the tides, which ought always to follow the moon, and flow from east to west: but, to allow the tides their full motion, the ocean in which they are produced ought to extend from east to west at least 90 degrees, or 6255 English miles; because, that is the distance between the places where the water is the most raised and depressed by the moon. Hence, it appears that it is only in the great oceans that the tide can flow regularly from east to west; and, hence, we also see why the tides in the Pacific Ocean exceed those in the Atlantic, and why the tides in the torrid zone, between Africa and America, though nearly under the moon, do not rise so high as in the temperate zones northward and southward, where the ocean is considerably wider. The tides in the Atlantic, in the torrid zone, flow from east to west till they are stopped by the continent of America; there the trade winds likewise continue to blow in

NATURAL CHANGES OF THE EARTH, &c. 81

that direction. When the action of the moon upon the waters has in some degree ceased, the force of the trade winds, in a great measure, prevents their return towards the African shores. The water thus accumulated* in the gulf of Mexico, returns to the Atlantic between the Island of Cuba, the Bahama Islands, and East Florida, and form that remarkable strong current called the Gulf of Florida.

CHAPTER VII.

Of the Natural Changes of the Earth, caused by Mountains, Floods, Volcanoes, and Earthquakes.

THAT there have always been mountains from the foundation of the world, is as certain as that there have always been rivers, both from reason and revelation ;† for they were as necessary before the flood for every purpose as they are at present. If the earth were perfectly level, (as some of our world-makers have imagined, though directly contrary to the Scripture) there could be no rivers, for water can flow only from a higher to a lower place; and instead of that beautiful variety of hills and valleys, verdant fields, forests, &c. which serve to display the goodness and beneficence of the Deity, a dismal sea would cover the whole face of the earth, and render it at best an habitation for aquatic animals only.

All mountains and high places continually decrease in height. Rivers running near mountains undermine and wash a part of them away, and rain falling on their sum

*To show that an accumulation of water does take place in the gulf of Mexico, a survey was made across the isthmus of Darien, when the water on the Atlantic was found to be 14 feet higher than the water on the Pacific side. Walker's Familiar Philosophy, Lecture xi.

+ Four rivers, or rather four branches of one river, are expressly mentioned before the flood, viz. Pison, Gihon, Hiddekel, and the Euphrates. Genesis, chap. ii. And in the 7th chapter of Genesis, at the time of the flood, we are told that the fountains of the great deep were broken up, the windows of heaven were opened, the waters prevailed exceedingly upon the earth, and all the high hills and mountains were covered.