Page:Philosophical Transactions - Volume 004.djvu/229



Oncerning my Thermoscope and Baroscope (the former of which is known to give accompt of the Temper of the Air, as to Heat and Cold; the other, of its Weight;) I have some few particulars to add to the Observations, I sent you some years agoe.

The first is, that, whereas I did then observe, that in Hot weather the Quick-silver in the Baroscope did use to rise observably, espec ally [sic] in Sun-shine and the Heat of the day; which might seem to argue the Air to be thereby made heavier, (which I find was the case of some other Baroscopes as well as of mine, and put us to some thoughts concerning the reason of it;) I do now find (having kept the same Barometer, for the space of five years, unalter'd,) the case, for these two years last past, to be somewhat otherwise: And that in hot Sun-shiny weather the Quick-silver doth rather subside a little; and in extreme Cold and Frosty weather it riseth.

Which maketh me Judge the Cause of these contrary Observations to be this, viz. That the Quick-silver, at its first putting into the Tube or Baroscope, was not so perfectly cleans'd from Air, but that some small quantity of it did remain, undiscern'd, in the Quick-silver: Which latent particles of Air, though so small as not to be at all discernable to the eye by bubbles, yet by the external heat (adding new strength, as it useth to do, to its Elastick or Springy power) were so much expanded as to give somewhat a greater bulk to the same quantity of Quick-silver, with which it was mingled, and consequently to make it rise somewhat higher, as being specifically lighter (that is, having the same weight in a larger dimension, or, in the same dimensions, a lesser weight;) and, upon the recess of the external heat, the Spring again slackening, the Air, being more compress'd, suffer'd the Quick-silver to be again contracted into its former lesser Dimensions, and so to become heavier, and not to rise so high as before, when it was hotter. But now, the Quick