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make the Air more cool and more elastick; by which Means the Waters are yet more encreased upon the fame: And that it may not be thought that this is inconfiftent with what has been faid above, of the Defcent of the Waters more strongly at Noon, from the Mountain called the Pike of Teneriff; let it be confider'd, what was farther faid about this Mountain, namely, that the Snow which cover'd the Top of the fame, being melted by the Heat of the Sun at Noon, caused the Waters to run down more violently at that Time.

I think that these Experiments may ferve for Proofs, that the Cold produced by the Shadows of the Mountains in the Air, may juftly be accounted one of the Reafons why fo many Watry Vapours are carried thither in a continual Stream.

SECT. XLII. Vapours fufficient to produce Rivers.

THE Only Difficulty that feems to remain, how there can afcend fo great a Quantity of Vapours as may fuffice to produce great Rivers: To anfwer which, we do not here pretend to maintain, that all Rivers proceed from thefe Vapours, or that they are the only Cause thereof; fince perhaps, according to the Opinion of others, the Sea entering into Subterraneous Caverns, may, by way of Filtration, leave its Salts behind it, and fo produce Sweet and Fresh Fountains: And befides, it may be, the Subterraneous Fires may cause these Waters that come from the Sea into the Cavities of the Earth, to exhale and afcend in Vapours, which being again turned into Water, by the Cold which they meet with above, may produce Fountains. But it is however a fufficiently probable Truth from what has been fhewn before, that the faid Vapours may juftly be reckon'd among the principal Causes of Ri

vers. Since the Sea, and other Waters exposed to the Sun, do continually tranfmit Vapours upwards, which, being collected upon the Mountains, and coming down again in Rain, Snow, or Hail, are proper to produce Rivers which may flow a long Time without ceafing, and fupply great Streams. This may be in fome manner inferr'd from the Obfervations of Mr. Mariotte in his Treatife du Movement des Eeaux (Engl Tranflation, P. 18) who fays, that at the lower end of a heap of Rubbish, which was about three Foot high, and whofe Superfices was about 500 French Fathoms (forafmuch as the Rain that fell upon it, and ran down upon it from the Tops of the neighbouring Houses, could not foak thro' by reason of the Hardness of the Ground) there was a continual little Stream of Water.

But the fame will be yet better fhewn hereafter, from the Calculation which the faid Mr. Mariotte makes, concerning the Warers of the Seine, compared with the quantity of Rain falling upon thofe Tracts of Land from whence this River has its Órigin. See the faid Treatife, English Translation, page 22, 23, 24.) by which it is proved, that in cafe there falls fo much Rain-Water every Year upon these Lands, that in case it remained there, would rife to the Heighth of 15 Inches, there would be 6 times as much as is requifité to run down the Seinė in one Year; and in cafe the Heighth of fuch RainWater fhould amount to 18 Inches, there would be 8 times as much; as likewife, if you fhould fuppofe it to rife to 20 Inches, there would fall g times as much Water upon thofe Places as flows thro' the Bed of the faid River.

SECT. XLIII. The Method of computing the Quantity of Rain Water falling in a certain Time.

THE Method which Mr. Mariotte makes use of to compute the Quantity of the said Rain-Water, is this; he took a fquare Veffel, which, for inftance was two Foot in Length, and as much in Breadth, which was raised upon a Horizontal Iron in fuch a manner that no Water could come into it, but what defcended immediately from the Sky into the Square of the Orifice thereof. This Water was conveyed by a Tube down into a round Veffel, from whence it would not be evaporated; fo, that by Gaging the Water in the faid round Veffel, it could be known, how high it would have risen from the Bottom of the Square Cistern : And fuppofing that there fell as much Water in one Year upon one Place, as upon another, one might compute pretty near the Depth of the Rain that would fall upon the circumjacent Land in the space of one Year.

SECT. XLIV. The Rain of Paris compared with that of Lifle.

Mr. Mariotte fays farther, that this Experiment having been made at Dijon, he found it to amount to 17 Inches; and another Perfon that tried the fame, computed it to 19 Inches 2 Lines. But they that defire to fee a very accurate Calculation and Comparison thereof, may find the fame in the Memoirs of the Royal French Acamedy, 1699. p. 25. for 6 Years following, one of which was made by Mr. Vauban at Lifle; and t'other by Mr. de la Hire at Paris, in the following Manner:

Years

And thus the Rain-Water that falls at Lifle eve ry Year, amounts to the Heighth of 20 Inches and 3 Lines, as that at Paris does to 20 Inches Lines, or at Medium of both, 21 Inches.

SECT. XLV. Rain-Water alone fufficient for

Rivers.

FROM hence, tho' it be plain, that there falls more Rain in one Place than in another, within the fame Space of Time, yet to make a general middle Computation, it may be fafely advanced, that there falls about 20 Inches of Rain yearly upon the Earth, and confequently 9 times as much as was neceffary to fill the River Seine in one Year. So that, tho' we should deduct from thence all that is ferviceable to other Ufes, and to the moiftening and fertilizing the Ground, and all that evaporates from it as foon as it is fallen; yet the Rain alone, without the help of other Vapours, furnishes Wa ter enough to maintain a far greater River than the Seine; which, if it happen'd in all Places of the Globe, and that many of these little Streams fhould be collected into one great and common Stream, they would together make up mighty RiHh 2

vers:

vers. Accordingly, we find by Experience, that by reafon of the Quantity of Waters which they bring with them, famous Rivers are produced by the Concourfe of feveral others that are leffer; which the Rain-Waters falling upon, many great Parts of the Earth, discharge therein.

SECT. XLVI. There is more Water in the Air than what defcends in Rain.

WE may now infer from what has been faid, that the Vapours which defcend in Rain only, seem to be more than fufficient to fupply the Rivers; but that, befides this, the Air does yet abound with a very great Quantity of Waters, may appear:

I. Because those Waters disclose themselves in Mifts, Dews, and. Nocturnal Moistures, and oftentimes do likewife defcend in invifible Vapours.

II. Thus we find in the making of the Hygrometers or Notiometers, or thofe Machines by which we measure the Moisture of the Air, as we do the Weight thereof by Barometers, and the Warmth by Thermometers; that the Strings of Mufical Inftruments, Ropes, Wood, and other Things, do undergo continual Changes by these Vapours floating in Air, according as they do more or lefs abound.

III. The Chymifts are particularly fenfible thereof, who, when they have reduced their lixivial Salts to pure and true Afcaline, as they call them, with all their Caution can hardly prevent them from being diffolved by the aforefaid moist Vapours.

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