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[[left-hand margin]] Barometer on the rise & fall of the Mercury therein. [[/left-hand margin]]

In assigning the cause of the rising and falling of the mercury in the Barometer, it is generally assumed and asked why does the quick silver fall in the Barometer when the air air is turbid and rendered heavier by various exhalations, and rise again when the air air is rendered lighter by clearing? Whereas every thing ought to happen quite contrary. [[strikeover]] The [[/strikeover]] In the question thus put, there is something assumed a supposed, which has nevery yet been proved, nor can easily be proved; viz, the air is rendered heavier, by being turbid & replete with various exhahations; and on the contrary lighter when it is clear. But the difficulties are removed by denying both these suppositions. Who can imagine that the motes in the circumabient air of a dark room were not present there before they were discovered by letting the light of the Sun, or that they retire as soon as this light is freely admitted? Let us omit [[insertion]] the [[insertion]] many experiments which have frequently been made with acid and alkaline salt, and select one which is more to our present purpouse.

[[left-hand margin]]
Quantity of Vapous do not add to [[insertion]] or depend upon [[\insertion]] the [[underline]] weight [[/underlined]] of the air.

Put the glass bell over the wet orb of the pnuematic engine, and, when the air pump begins to work, some light clouds or vapours will arise in the bell, (as related in the account of refraction, p.9 & 10,) which immediately [[strikethrough]]subside [[\strikethrough]] disperse & disappear upon the admition of the air. These exhalations could not enter the bell, when they became conspicuous, by diminishing the elastic power of the air. They also existed there after they disappeared, & were hidden in the pores of the air, which again sustains them by becoming heavier & more elastic. Or, those exhalations are present before they become visible by approaching each other; nor are they annihilated, or no longer exist in the air, after they disappear by being dissipated and thereby too subtle for our sight. They begin to approach one another, when the elasticity of the air is so far diminished as not to be able to sustain them; and they again recede & cease to affect our sight, when the former gravity + elastic force of the air is restored. Whence it appears, that these [[underlined]] two [[/underlined]] things happen [[underlined]] at the same time [[/underlined]], viz. a diminution of the elastic force of the air and the arising of exhalations, which [[strike through]] before hung [[/strike through]] were before dispersed in, and sustained by it, but are now gradually loosed [[insertion]]from the air [[\insertion]], and become visible by coagulating among themselves: also The air recovers it's elasticity and the vapous hanging in it are dissipated and disappear; tho' the [[underlined]] one [[/underlined]] cannot be said to be the [[underlined]] cause [[/underlined]] of the other: nor can the air be said to be heavier or lighter at one time than another, on the account of these exhalations.

The same happens in the air which surrounds our earth. Let it's elastic force be diminished, by any means whatsoever (of which hereafter) the dispersed & suspended exhalations necessarily subside & become visible, and when by