of the earth, the equator will be divided into two equal parts; consequently, the days and nights at the equator are always equal. Thus, the different seasons are clearly accounted for, by the inclination of the axis of the earth to the plane of its orbit,* combined with the parallel motion of that axis. CHAPTER V. Of the Origin of Springs and Rivers, and of the Saltness of the Sea. VARIOUS Opinions have been held by ancient, as well as modern philosophers, respecting the origin of springs and rivers; but the true cause is now pretty well ascertained. It is well known that the heat of the sun draws vast quantities of vapour from the sea, which, being carried by the wind to all parts of the globe, and being converted by the cold into rain and dew, it falls down * In addition to these observations, the author farther illustrates the seasons of the year by an orrery; and sometimes by a brass wire supported on two stands of different heights, correspondent to the diameter of the wire circle and the obliquity of the ecliptic; as in Ferguson's Astronomy, chap. x. But, as this last method does not so clearly shew the obliquity of the axis of the earth to the plane of its orbit take a board of any convenient dimensions, suppose two feet across, on which describe a circle, or an ellipsis differing little from a circle, draw a diameter O FO (Plate III. Fig. 1.) and parallel to this diameter let several lines ef be drawn, then bore several holes perpendicularly down in the points e, e, &c. of the circumference of the circle, take two pieces of wire crossing each other in an angle of 230 28'; as a g, and nf, of which a g the perpendicular wire is the longer, and connect them by a straight wire ef; then placing a small globe on the point n, and a light in the centre of the circle, of the same height as the centre of the little, globe; let the point g in the longer wire be fixed successively in the holes e, e, &c. in the circumference of the circle, so that the base ef of the wire, may rest on the lines ef in the plane of the earth's orbit, the seasons of the year will be agreeably and accurately illustrated. If the little globe be placed upon the point a, instead of the point n, and the same method be observed in moving the wires round the orbit, there will be no diversity of seasons. The diurnal revolution of the earth may be shewn by moving the globe round the wire nf, as an axis with the finger. upon the earth; part of it runs down into the lower places, forming rivulets, part serves for the purposes of vegetation, and the rest descends into hollow caverns within the earth, which, breaking out by the sides of the hills, forms little springs; many of these springs running into the valleys increase the brooks or rivulets, and several of these meeting together make a river. Dr. Halley* says, the vapours that are raised copiously from the sea, and carried by the winds to the ridges of mountains, are conveyed to their tops by the current of air; where the water being presently precipitated, enters the crannies of the mountains, down which it glides into the caverns, till it meets with a stratum of earth or stone, of a nature sufficiently solid to sustain it. When this reservoir is filled, the superfluous water, following the direction of the stratum, runs over at the lowest place, and in its passage meets perhaps with other little streams, which have a similar origin; these gradually descend till they meet with an aperture at the side, or foot, of the mountain, through which they escape and form a spring, or the source of a brook or rivulet. Several brooks or rivulets, uniting their streams, form small rivers, and these again being joined by other small rivers, and united in one common channel, form such streams as the Rhine, Rhone, Danube, &c. Some springs yield always the same quantity of water, equally when the least rain or vapour is afforded, as when the rain falls in the greatest quantities; and as the fall of rain, snow, &c. is inconstant or variable, we have here a constant effect produced from an inconstant cause, which is an unphilosophical conclusion. Some naturalists, therefore, have recourse to the sea, and derive the origin of several springs immediately from thence, by supposing a subterraneous circulation of porcelated waters from the fountains of the deep. That the sun exhales as much vapour as is sufficient for rain is past dispute, having been several times * Philosophical Transactions, No. 192. proved by actual experiments. Dr. Halley* determined by experiment and calculation,† that in a summer's day, there may be raised in vapours from the Mediterranean 5280 millions of tuns of water, and yet the Mediterranean does not receive from all its rivers above 1827 millions of tuns in a day, which is little more than third part of what is exhausted by vapours ; and from the river Thames, twenty millions three hundred thou sand tuns may be raised in one day in a similar manner. In the Old Continent there are about 430 rivers which fall directly into the ocean, or into the Mediterranean and Black Seas, and in the New Continent, scarcely 180 rivers are known, which fall directly into the sea: but in this number, only the greater rivers are comprehended. All these rivers carry to the sea a great quantity of mineral and saline particles, which they wash from the different soils through which they pass, and the particles of salt, which are easily dissolved, are conveyed to the sea by the water. Dr. Halley imagines that the saltness of the sea proceeds from the salts of the earth only, which rivers convey thither, and that it was originally fresh. So that its saltness will continue to increase; for, the vapours which are exhaled from the seas are entirely fresh, or devoid of saline particles. Others imagine that there is a great number of rocks. of salt at the bottom of the sea, and from these rocks it acquires its saltness. Some writers again, have imagined that the sea was created salt that it might not corrupt; but it may well be supposed that the sea is preserved from corruption by the agitations of the wind, and from the flux and reflux of the tide, as much as by the salt it contains; for, when sea water is kept in a barrel, it corrupts in a few days. The Honourable Mr. Dr. Halley was an eminent mathematician, astronomer, and philosopher, born in London in the year 1656. Philosophical Transactions, No. 212. As evaporation cannot carry off fixed salts, it would appear that if the above calculation be accurate, the Mediterranean would be more salt than the Ocean, but it must be remembered that a current sets constantly out of the Atlantic Ocean into the Mediterra nean. Buffon's Natural History. Boyle* relates, that a mariner becalmed for thirteen days, found at the end of that time the sea so infected, that if the calm had continued, the greater part of his people on board would have perished. The sea is nearly equally salt throughout, under the equinoctial line and at the Cape of Good Hope, though there are some places on the Mozambique coast where it is salter than elsewhere. It is also asserted, that it is not quite so salt under the arctic circle as in some other latitudes,f this probably may proceed from the great quantity of snow, and the great rivers which fall into those seas: to which we may add, that the sun does not draw such quantities of fresh water, or vapours, from those seas as in hot countries. It is worthy of remark, that all lakes from which rivers derive their origin, or which fall into the course of rivers, are not saline;‡ and almost all those, on the contrary, which receive rivers, without other rivers issuing from them are saline: this seems to favour Dr. Halley's opinion respecting the saltness of the sea, for evaporation cannot carry off fixed salts; and, consequently, those salts which rivers carry into the sea remain there. It is asserted to be the peculiar property of seawater, that when it is absolutely salt it never freezes ;§ and that the islands or rocks of ice which float in the sea near the poles, are originally frozen in the rivers, and carried thence to the sea by the tide; where they continue to accumulate by the great quantities of snow and sleet which fall in those seas. According to this opinion, great quantities of ice can be produced only from great quantities of fresh water, or from large rivers, and as large rivers can only flow from large tracts of land, it would appear that there must be immense * A younger son of the Earl of Cork, and one of the most celebrated philosophers in Europe, born at Lismore in the county of Waterford, 1626-7. See his treatise on the saltness of the sea, published in 1674. In a treatise on Chemistry, published by Dr. Thompson of Edinburgh, page 375, it is stated that the ocean contains most salt between 10 and 20° south latitude, and that the proportion of salt is the least in latitude 57° north. Buffon's Natural History, Chap. II. * tracts of land near the south pole, for the Antarctic Ocean abounds with fields or mountains of ice, as well as the Arctic Ocean; but our circumnavigators have traversed the southern Ocean to upwards of seventy degrees south latitude, without discovering any land. With respect to the freezing of salt water, we have several instances of the Baltic, and other seas, being frozen over, when the ice on the surface could never proceed from rivers. It is true that the sailors frequently take large pieces of the rocks of ice, and thaw them for the use of the ship's company, and always find the water fresh; but it does not follow from this that the ice is formed in the river. As fresh water only iş extracted from sea water by the heat of the sun, and carried into the atmosphere: may not the fresh, without the saline, particles of sea water be converted into ice by extreme cold? CHAPTER VI. Of the Flux and Reflux of the Tides. A TIDE is that motion of the water in the seas and rivers, by which they are found to rise and fall in a regular succession; and this flowing and ebbing is caused by the attraction of the sun and moon.† Suppose the earth to be entirely covered by a fluid, as A, B, Z, C, D, Q, N. (Plate III. Fig. 2.) and the action of the sun and moon to have no effect upon it; then * The Baltic Sea is not so salt as the Ocean, and the proportion of salt is increased by a west wind, and still more by a north-west wind: a proof that not only the saltness of the Baltic is derived from the ocean, but that storms have a much greater effect upon the waters of the ocean than has been supposed. Dr. Thomson's Chemistry, page 375. The Baltic Sea has little or no tides, and a current runs constantly through the Sound into the Cattegate sea. This was known to the ancients. Pliny expressly says, that the cause of the ebb and flow is in the sun, which attracts the waters of the ocean; and that they also rise in proportion to the prox imity of the moon to the earth. Dr. Hutton's Math. Dict., word Tides. |