ses into that rarefied place; for the same reason which causes it to ascend into the exhausted tube, and forms the water-spout or pillar of water in the air. The water-spouts generally break about their middle, and the falling waters occasion great damage, either to ships that have the misfortune of being under them, or to the adjoining land; for such spouts are sometimes formed on a lake, or river, or on the sea close to the land.

As the motion of the air has a greater or lesser velocity, the wind is stronger or weaker; and it is found from observation, that the velocity of the wind is various, from the rate of 1 to 100 miles per hour.

The following particulars respecting the velocity, &c. of the wind are extracted from a table which appeared in the 51st volume of the Philosophical Transactions, by Mr. J. Smeaton, the celebrated engi

neer.

When the velocity of the wind is one mile per hour it is hardly perceptible. From 2 to 3 just perceptible.

4 5 gentle pleasant wind, or breezes. 10-15 pleasant brisk gale.

20-25 very brisk.

30-35 high winds,

40-45 very high.

50 miles per hour a storm or tempest.

60

80

100

a great storm.

a hurricane.

a hurricane that tears up trees carries buildings before it, &c.

The winds are of immense and indispensable use. Besides their more obvious effects in driving of ships, windmills, &c. they preserve, by mixing, the necessary purity of the air. The winds, likewise drive away vapours, clouds fogs, and mists from those parts in which they are copiously formed, to others which are in want of moisture; and thus the whole surface of the earth is supplied with water. It is the winds which diminish the heat, and augment the moisture of the torrid zone, and produce contrary effects on those of the polar regions, so as to render those districts of the globe, which the ancients deemed totally unfit for the abode of man, and other animals, by reason of excessive heat, not only habitable, but salutary and pleasing to man and beast, and yielding great variety and abundance of the choice productions of nature.

CHAP. VIII.

Of Springs, Rivers, and the Sea.

HAVING viewed water as it takes its departure from the bosom of the deep and forms the watery meteors, we shall now survey it as it rises in the salient spring, and gives birth to the gurgling rill, or uniting, gives coolness to the landscape in the magnificent stream, that in its ample range fertilizes its neighbourhood.

Various have been the theories, or rather hypotheses relating to the origin of springs; but it seems the general opinion of those who have made this branch of natural philosophy their study, that the true principles which supply the waters of fountains or springs, are melted snow, rain water, and condensed vapours.

*

The prodigious quantity of vapours raised by the sun's heat, and otherwise, being carried by the winds over the low lands to the very ridges of mountains, as the Pyrenean, the Alps, the Apennine, the Carpathian, in Europe; the Taurus the Caucasus, Imaus and others in Asia; Atlas the Montes Lunæ, or mountains of the moon, with other unknown ridges in Africa; the vapours being compelled by the stream of air to mount up with it to the top of those mountains, where the air becoming too light to sustain them, and condensed by cold they strike against their summits, which causes an union of their particles, and are precipitated in water, which gleets down by the crannies of the stone; and entering into the caverns of the hills, gathers, as in an alembic, into the basons of stone it finds, which being once filled, all the overplus of water that comes thither, runs over by the lowest places, and breaking out by the sides of the hills forms single spings.

Many of these springs running down by the vallies, between the ridges of the hills, and coming to unite, form little rivulets, or brooks; many of these again

L

meeting in one common valley, and gaining the plain ground, being grown less rapid, become a river; and many of these being united in one common channel, make such enormous streams as the Rhine, the Rhone and the Danube. And it may almost pass for a rule, that the magnitude of a river, or the quantity of water it discharges, is proportional to the length and heights of these very ridges from whence the fountains arise.

The several sorts of springs observed are common springs, which either run continually, and then they are called perennial springs; or else run only for a time, or at certain times of the year, and then they are called temporary springs. Intermitting springs, or such as flow and then stop, and flow and stop again, by regular alternations or intermissions. Reciprocating springs, whose waters rise and fall, or flow and ebb, by regular intervals, or reciprocations of the surface.

If those reservoirs of water in the body of mountains be situated where mineral ores abound, or the ducts or feeding streams run through mineral earth, it is easy to conceive the particles of metal