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silver, having continued in the Tube for five years and upwards, hath by its own weight cleansed it self better from that little Air that was in it; and that Air, freed from its entanglement with the Quick-silver, being got up into the void part of the Tube above the Quick-silver, doth act contrary-wise, that is, when it is by Heat (upon the strengthening of its Spring) expanded, it presseth down-ward upon the Quick-silver, and doth a little depress it; and, on the contrary, when by Frost or very Cold weather this Air (by the abatement of its Spring) is contracted, the Quick-silver, freed from that pressure, riseth a little. But the Rising and Sinking on this accompt, (as well that formerly, when this Air was in the Quick-silver, as that now, when it is gotten above it,) is not very considerable; hardly exceeding the twelfth part of an inch, or thereabout.
This Accompt I thought not amiss to give you, because it may possibly keep your self, or some others who make use of Baroscopes, from being impos'd upon by such Observations; as if the Sun-shining or Heat of the weather did make the Air heavier than before, whereas it seems to be but an accidental operation upon that un-observ'd Air latent in the Quick-silver. And I thought it the more necessary to take notice of it, because it is not so easily discover'd, unless by keeping the Baroscope unalter'd for a longer time, than (perhaps) is done by some of those, who make use of it; and, without which, I had not discern'd it my self.
On this occasion, I shall add another Accident, which I lately took notice of. Observing in the late hard Frost, that the Quick-silver did not rise a little, at such time as, by reason of the fierce freezing, I expected it might; I did suspect (as it prov'd to be,) that a little drop of water, (which was at first made use of for the cleaning of the Quick-silver from the Air, and which hath ever since remain'd on the top of the Quick-silver within the Tube,) was frozen fast to the glass, so as to stop the Quick-silver from ascending: And, to try, whether it were so or no, I did a little shake the Tube by moving it up and down, so as to make the Quick-silver undulate. Whereupon I found the frozen drop of Water to keep its place, while the undulating Quick-silver did several times beat against it. And,(which
