And I have often thought, when I found good Salt of Tartar within Doors, and in a Laboratory, turned into a clear Liquor, that there must be a very great quantity of these invifible Vapours continually floating in the Air, fince, that in fo small a Place, in fo fhort a Time, and in fo little an Orifice or Mouth, as that of the Glafs containing the faid Salt, there could be fo much Water gather'd together; for which reafon Mr. de la Hire himfelf (Jee the Memoirs of the Royal Academy, for 1703. p. 78) feems to fufpect, that Stones likewife, in which there were found any Salts proper to draw Waters to them, might ferve to collect the like Vapours into Springs or Rills: at leaft, the Experiment which he there relates, that even in Places where it does not Rain at all, as in a Cellar, for instance, a confiderable quantity of moift Vapours may be found.

V. But this appears yet more plainly from what the faid Mr. de la Hire, p. 77. fays farther, that there were a great many Experiments made, by which it was proved, that if you fet a very large Veffel with Water in the Air, there will much more Wa ter evaporate out of it than can defcend from the Air upon the like Breadth.

SECT. XLVII. Exhalations from Canals and Ditches.

To make a rough Guefs thereof, a certain curi ous Miller, whom I asked, How much he thought the Water in the Meadow where his Mill was could be diminished in one Day by the Heat of the Sun? Anfwer'd me, that in a very warm Day there was (to fpeak within Bounds) at leaft the quantity of an Inch in Depth, efpecially, if the hot Weather continued any time, and by that means his Waters could not be much encreafed by thofe that ran Hh3 down

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down from the Lands about him; for otherwife it did not appear to him, that he loft fo much Water: But those who have ever seen how much Water is exhaled from the Canals or Ditches in a very little space of Time, efpecially when the Ground is dryed by a continual Heat, will not judge, that we exceed the Truth, in allowing an Inch a Day in very hot Weather.

For this purpofe, in the beginning of June, 1710, I filled a flat Earthen Pan with Water, and fet it in the open Air in a bright and clear Day, and examining it after four and twenty Hours, I found that there was a full Inch loft in the Depth of it, by Exhalation.

Now, if we fuppofe, that the Evaporation of all the Waters throughout the whole Earth be equally great, and amounts to an Inch a Day, according to this Calculation, there would be every Year 365 Inches in Depth drawn into the Air: All which, fuppofing it to fall down again in Rain, would be capable of overflowing the whole Superficies of the Earth 365 Inches high in one Year.

SECT. XLVIII. Experiments to fhew, that Evaporations are likewife performed by Cold.

LET it not be objected against us, that there cannot be fo much Water exhaled under and near the Poles, by reafon of the Coldnefs of thofe Parts of the World, becaufe,

1. In the very fharpeft Frofts, Vapours do continually afcend from our Canals and Ditches upon breaking the Ice: Now in order to enquite whether this, as fome think, might likewife proceed from the Subterraneous Heat; upon the 14th January, 1709, which, as every Body knows, was a violent and uncommon Froft, I took

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an Earthen Bason, and pouring 40 Ounces of Water into it, put it into Scales in a Room where there was no Fire at all made, and found that upon its freezing, it had loft in 17 or 18 Hours, above a quarter of an Ounce in Weight; having taken care to prevent the breaking of the Veffel upon the Waters freezing, by keeping a little Hole in the middle of the Ice always open; thro' which the Water being continually preffed from under the Ice, it made a great Convexity and Protuberance above the Superficies of the faid Ice; a fure Sign that Water is both moved and rarified even by Cold. And fo before, upon the 8th of January, in the fame Year, a quantity of Snow being put into the Scales, fuffer'd a vifible Diminution of its Weight; notwithftanding that it had fallen above three Days before, and lain all the time in the Air; and that which is more, we find even Ice itself will evaporate in the coldest Nights; as has been likewife obferved by Mr. Boyle, in his Book, de Athmofphær. Corp. Confift.

And whilft I am writing this, a Person that has been one and twenty times in Greenland, tells me, that when the Weather is Calm, and without Wind, the Sea frequently fmoaks and emits a vifible Steam, which Varenius does alfo confirm, p. 361, where, fpeaking of the Seafons in the Frigid Zone, he fays, that a heavy, foggy and thick Vapour floated over the Water, infomuch that Peoples Sight was intercepted thereby : From all which it follows, that a great quantity of Vapours afcends from Water in the coldeft Regions of the World.

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SECT. XLIX. Objections Answer'd.

Now if it fhould be fuppofed and granted, that the Waters produced by the Exhalations of thefe Vapours, do not near amount to the quantity of an Inch a Day, as in our warm Climates; we may fet againit it, that the Heat of the Southern Regions, quite as far as the Equator, is much greater than that of ours. And again, that the Superficies of the Earth between us, who lye in about the Latitude of 52 Degrees, and the North Pole, is much fmaller than that which is between the Parallel inhabited by us, and the Equator: Wherefore, the Parts of the Earth, where the Air is much hotter than with us, are incomparably larger than thofe where it is much colder. So that we fhall not feem to have made any great Mistake in taking the quantity of our own Exhalations, or that of an Inch a Day, for a Medium common to the whole Superficies of the Earth.

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But, forafmuch as the Terreftrial Globe is not cover'd with Water all round, let us, for greater Certainty, fuppofe farther, that the Seas, Rivers, and Lakes, do take up no more than half the Superficies thereof: Then the Vapours that daily attracted, to the quantity of one Inch in depth, will cover the whole Face of the Earth, when they defcend in one Year no more than the half of 365, that is to fay, only 182 Inches deep....

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SECT. L. A Calculation after the Rate of an HalfInch daily Exhalation.

Now if the daily Exhalation of one Inch fhould appear too large a Computation, let us fuppofe is to be half as much.

This feems to be the more probable, for the fol lowing Reafons: First, Because Dr. Halley (by keeping a Thermometer with Salt-Water con tinually in that Degree of Warmth in which he had obferv'd the Air to be in a hot Day) found upon Trial, and by Weight, that the Superficies of that Water was in the Space of two Hours fals len is of an Inch, that is in 12 Hours, or in 25 Hours of an Inch, fuppofing the Exhalation to be always equal.

And again; Forafmuch as the above-mention'd Miller had, at my Requeft, with great Exactness obferved, that from Tuesday the 7th of June, 1712, to the fame Hour the following Fryday the 10th, the Water in the Purmer-Meer, or Lake, had loft of its depth 1 Inch; that is to fay, every Day half an Inch, tho' the Weather was then much cooler than the preceding Days; and he would have proceeded farther in these Obfervations, if the Weather had not begun to be rainy and windy. After which, the Air being again warmer and calmer, he informed me, that in three other Days there were evaporated two compleat Rynland Inches, which is fo much more than half an Inch a Day; and therefore, if we fuppofe the quantity of Exhalations to amount to no more than half an Inch every Day, and the Superficies of Land and Water to be exactly equal to one another, the Rain that will fall upon the whole Earth, will amount to the half of one 1814, that is about 90 Inches