In thick foggy weather, I find my Quicksilver to rise; which I adscribe to the heaviness of the Vapours in the Air. And I have never found it higher, than in the foggy weather above-mentioned.
In Sunshiny weather it riseth also (and commonly the clearer, the more;) which, I think, may be imputed partly to the Vapors raised by the Sun, and making the Air heavier; and partly to the Heat, increasing the Elastick or Springy power of the Air. Which latter I the rather add, because I have sometimes observed in Sunshiny weather, when there have come Clouds for some considerable time (suppose an hour or two) the Quicksilver has fallen; and then, upon the Suns breaking out again, it has risen as before.
In Rainy weather, it useth to fall (of which the reason is obvious, because the Air is lightned, by so much as falls:) In Snowy weather, likewise, but not so much as in Rain. And sometimes I have observed it, upon a Hoar-frost, falling in the night.
* The Author of these Observations intends hereafter more particularly to observe, from what points those Winds blow, that make the Quicksilver thus subside.
For Windy weather, I find it generally to fall; and that more universally, and more discernably, than upon Rain: (which I attribute to the Winds moving the Air collaterally, and thereby not suffering it to press so much directly downwards: the like of
which we see in swimming, &c.) And I have never found it lower than in high Winds.*
I have divers times, upon discerning my Quicksilver to fall without any visible cause at home, looked abroad; and found (by the appearance of broken Clouds, or otherwise) that it had rained not far off, though not with us: Whereupon, the Air being then lightened, our heavier Air (where it rained not) may have, in part, discharged it self on that lighter.
A more particular Account of those Observations about Jupiter, that were mentioned in Numb. 8.
Since the publishing of Numb. 8. of these Transactions, where, among other particulars, some short Observations were set down touching both the shadow of one of Jupiter's Satellits, passing over his Body, and that Permanent Spot, which manifests the Conversion of that Planet about his own Axis; there is come to hand an Extract of that Letter, which was written from Rome, about those Discoveries, containing an ample and particular Relation of them, as they were made by the Learned Cassini, Professor of Astronomy in the University of Bononia. That Extract, as it is found in the French Journal des Scavans of Febr. 22. 1666. we thus English.