indeed that very Comet, which he saw with his naked Eye, and observed with his Geometrical Instruments, the said 12, 13, and 14. dayes of February; or whether it was another, and whether he had lost that Comet, which moved towards the Second Star in Aries: but leaves it to the Learned World, and particularly to the Royal Society, after they shall have well examined and considered all his Observations, and the Calculus raised therefrom, to judge of this, and the other particulars in controversie.
II. Isaacus Vossius de NILI et ALIORUM FLUMINUM ORIGINE. It was Numb. 14. of these Transactions, that gave an account of the Cause of the Inundation of the Nile, as it was rendred by Monsieur de la Chambre: This is to give you another, not only of the Inundation, but also of the Origine of that, and of other Rivers, as it is delivered by Monsieur Isaac Vossius, who undertakes in this Book to shew;
1. That those Subterraneous Channels, through which several Philosophers teach, that the Sea discharges it self into the Rivers, are not only imaginary, but useless, in regard 'tis impossible for the water to rise from the Subterraneous places up to the Mountains, where commonly the Sources of Rivers are.
2. He explicates, why, if a Pipe be put into a Bason full of Water, the water is seen more raised in the Pipe, than in the Bason, and rises higher according as the Pipe is narrower; On the contrary, if the same Pipe be put into a Bason full of Quicksilver, the Quicksilver stayes lower in the Pipe, than in the Bason. The reason, which he renders hereof, is, That as the Water sticks easily to all it touches, it is sustain'd by the sides of the narrow Pipe wherein it is included: And indeed, if the Pipe be quite drawn out of the Water, the Water doth not all fall out, but so much of it remains, as the sides of the Pipe could sustaine: Whence it is, that the Water which is kept up by the Walls of the Tube, weighing no longer upon that which is in the Bason, is thrust upwards, and keeps it self raised above its Levell; but the Quicksilver not adhering so easily, as Water, to Bodies it touches, is not sustained by the sides of the Tube, and so mounts not above its Levell, but rather descends below it, because the Pipe, which is streight, hinders the endeavor that is in the Mercury to rise to its Level. He adds, that this Observation makes nothing for the Explication of the Origine of Rivers; because, though it be true, that the Water
by this means rises above its Levell, yet it does never run out at the top of the Pipe. Having said this, he answers to the other Arguments, commonly alledged to maintain this Opinion.
3. He pretends, that all Rivers proceed from a Colluvies or Rendevous of Rain-waters, and that, as the Water, that falls upon Hills, gathers more easily together, than that which falls in Plaines, therefore it is, that Rivers ordinarily take their Source from Hills. Thence also comes it (saies he) that there are more Rivers, than Torrents, in the Temperate Zones; and, on the contrary, more Torrents, than Rivers, in the Torrid Zone: For, as in hot Climats the Mountains are far higher, the Water, that descends from them with impetuosity, runs away in a little while, and formes such Collections of Water, as soon dry up, but in cold Climats, the Waters do not run away but slowly, and are renew'd and recruited by Rain, before they are quite dryed up; because the Hills are there lower, and so the Bed of Rivers hath lesse declivity.
Having thus discoursed of Rivers in General, he treats of the Nile in particular; and there
1. Observes, That the Order of the Seasons of the Year is quite inverted under the Torrid Zone. For, whereas it should be then Summer, when the Sun is near; and Winter, when the Sun is farther off: Under the Torrid Zone 'tis never lesse hot, than when the Sun is nearest; nor more hot, than when the Sun is farthest off: So that to the people that live between the Æquinoctial and the Tropicks, Summer begins about Christmass, and their Winter, about St. Johns day. The reason whereof is, (saith he) that when the Sun is directly over their Heads, it raises abundance of vapors, and draws them so high, that they are presently converted into Water by the coldnesse of the Air; whence it comes to passe, that then it rains continually, which does refresh the Air; but when the Sun is farther off, there falls no more rain, and so the Heat becomes insupportable.
2. He proves by many recent Relations, that the Sources of the Nile are on this side of the Æquinoctial in Æthiopia, of which he gives a very accurate Mappe, correcting many faults which Geographers are wont to commit in the Description of the Kingdom of the Abyssins, which they believe to be much greater than indeed it is.