perficies, as is evident from the Ray Lm, which comes from the western Air to L; for if it came from mas from a Concavity, it's incident Ray should have come from the Place x, whereas it comes from o, or between f and o. Therefore the Ray Lm fo reflected, is from the Particle m as the Fig re required. And in the Air there are Particles of very different Figures; and fo no wonder if they make Reflections thro' the Air every way.

PROPOSITION XXXIX.

Suppofing the Twilight is not made by one but a double Reflection, to find from thence the Altitude of the Air, which may agree better with other Obfervations.

in

IT was faid in the laft Propofition, that the Ray gbf, (Fig. 29.) which makes the first reflected Ray in the Beginning of the Twilight, does not come from the Sun itself, but that 'tis reflected g; let therefore the incident Ray be gl (which may touch the Earth m p, and fo lg is the first Ray which can come to g) and let us now fuppofe it to come from the Sun itself immediately, and by Refraction to be turned a little afide; that is, let QL be the Ray from the Sun, and let lpg be the refracted Ray, and gb s the reflected Ray, and fL the fecond reflected Ray. The Altitude of the Air Tƒ is to be found; and, because the incident Ray Q refracts into glx, let us fuppofe the Angle of Refraction glx to be 30 min. and that the Center of the Sun is 17 degr. under the Horizon, when the Twilight begins; therefore the Limb of the Sun will be 16 degr, 45 min. under it, and fubftracting the 30 min. for the Refraction, and Angle n Kx will be 16 degr. 15 min. which is the Depreffion of the Sun's Limb after Refraction, And because

KL, Kp are equal, and also fL, gp, then Kg, Kf will be equal, and the Angle K fg equal to Kgf, and both together equal to g Kn 16 degr. 15 min. therefore Kfg is 8 degr. 7 min. and fTL 4 degr. and TfL 80 degr. whence Tf is found to be 861 Miles and a half, and thus the Air's Altitude 1 Mile and a half, which is far lefs than was formerly made by the Twilight, and will be found much lefs if a threefold Reflection be made the beginning of the Twilight; which is not impoffible. And this double or triple Reflection is better allowed for the Caufe of the Twilight's Continuance, than that which Kepler brings from the fplendid Matter about the Sun. You may fee more of the Time of Twilight and it's various lengths, in the fecond Part of this Book.

PROPOSITION XL.

To find the Altitude of the Clouds by the Quadrant (a).

THE Air being calm and clear, fix on fome Point in the Cloud that is remarkable, and mea

(a) Mr Boyle tells us, that a you found the Clouds fo low, good Aftronomer, who had because I have often found it divers times measured the • fo. I remember that I conHeight of the Clouds, affured trived a way, about two or him, that he could never find 'three years ago, of taking the any that were above three quar- Height of the Clouds with ters of a Mile high, and that a Quadrant, all at one Station; few exceeded half a Mile. And and I never could find any, Mr Crabtrie (an excellent Ma- that were at molt above a thematician of the laft age) upon Mile and half high.' Aftermeasuring their Height, was wards, faith he, I found the furprized to find them no high-fame method in Kepler (Aftr. er, and wrote to his friend (that' great Genius of the last age) Mr Horrox, about it. Who in his Letter, Nov. 23. 1637, tells him, I am not furprized that

Cop. p. 70.) where he faith, The Clouds are never above 6 a quarter of a German Mile high, that is one of our English Miles.

Hh 4

fure

SECT. VI. fure it's Height as if it were the Top of a Steeple, at two Stations: by two Obfervers at the fame time; and thus you may find it's Height, which is never found to be above a Quarter of a Mile.

PROPOSITION XLI.

To measure the Quantity of the Air, having it's Altitude given.

THIS is only to meafure the Space between the Earth and the utmost Bounds of the Air, which is easily done, having the Altitude of the Air, by measuring the Solidity of the Sphere made of the Earth and Air, and then of the Earth only; and fubftracting the one from the other, there remains the Quantity of Air.

PROPOSITION XLII.

The Air of certain Places bath fome peculiar Properties.

THUS in Egypt it feldom, or rather never, rains; and if small Rain fall at any Time, there follow Difeafes, as Catarrhs, Fevers, Afthmas, &c. The Inundation of the Nile, and the daily Hoạrfrofts in the Morning, fupply the want of Rain. And fo in the Kingdom of Peru, there are never feen Rains and in feveral Places under the Equator it rains for a whole half Year, and is fair the other. See Part ii. Chap. xxvi.

THE Iland Pulon Timor is for the most part covered with Mifts and Hoar-froft.

IN the Island Sumatra, the Air is unwholesome, on account of the several standing Pools in it; and the like holds of feveral other Places, as in old Mexico, Malacca, &c.

THE

THE Island of St Thomas, lying under the Equator, is thought of all Countries to have the groffeft unwholfomeft Air, tho' it be most fruitful in every kind.

IN the Province of Chili the Air is fo very fine and fubtile that the Blade of a Sword, sheathed without wiping, will not ruft.

IN the Azores the Air and Winds are so sharp as in a fhort time to corrode Plates of Iron and. the Tiles on the Houses, reducing them to powder.

ARISTOTLE fays, that on Mount Olympus there is not the leaft Motion of the Air, nor even any Air at all, for the Characters, written on the Duft there, remain as at firft, after many Years; and they that go up it, cannot live there, except they carry with them wet Sponges, by the help of which they breathe.

IN America, when the Spaniards were paffing from Nicaragua to the Province of Peru, many of them, as they paffed over the Tops of the Mountains, did, with their Horfes, there breathe their laft, or were turned into Statues with the Cold; and thus continued 'till they that escaped returned. Some think this was owing to want of Air, but that is not likely; nor what Ariftotle fays of Olympus; for the contrary is found on higher Mountains, whofe Tops are covered with Snow. Therefore 'tis certain thefe Mountains could not be above the Air, but the Air fnowed upon them. See the Chap. of Mountains. Bufbequius, an Eye-witness, fays, that Olympus in Summer is covered with Snow.

THE Air about the Islands in the Indian Ocean is fo fragrant with the smell of Spices, that Seamen perceive it (especially when the Spices are ripe) three or four Miles off, when the Wind is against them.

THE Sea Air is more unwholsome than that on Land, and lefs agreeable to thofe that are not used

to

to it. The Difference is very sensible when Seamen come near the Shore; for they know when they are within a Mile of the Land, by drawing in the Land Air in breathing. This the Seamen of Soffala, on the eaftern Shore of Africa, know efpecially.

6

WHILST this was printing, I met with an Obfervation made by David Fralichius on Mount Carphathus in Hungary, which, because 'tis very useful in forming a Judgment of the Altitude of the Air, and of it's feveral Regions, I thought proper to add it here, tho' it should have come in at Prop. xviii. He fays Carpathus is the chief of the Mountains in Hungary, which Name is common to all that Tract of the Sarmatian Mountains, which feparates the Hungarians from the Ruffians, Polanders, Moravians, Silefians, and those in that Part of Auftria beyond the Danube. Their high • and frightful Tops that are above the Clouds appear at Cæfareopolis. They are called fometimes by a Name importing that they are almost continually covered with Snows; and by another Name that imports them to be bald and fhaven • as it were. And indeed the Rocks there do far exceed the Alps in Italy, Switzerland, and Tyrol, in roughness and Precipices: they are almost unpaffable, and no Body goes near them but those that are curious Admirers of Nature.

..

AND to mention this by the way; when I was a Youth, having, in June 1615, a Defire to try how high thefe Mountains might be, I went up with two of my School-Fellows: when I had got up to the Top of the firft Rock with great Difficulty, and thought I was on the Top of all, there appeared another ragged Rock much high6 er; and when I had clamber'd to it, over many large and loofe Stones, (any one whereof being thrown down would carry fome hundreds far ⚫ greater