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153  by any means it is again restored, these vapors and clouds are oifsipates and vanish.  But at the time, or, for the same reason, before those subsiding vapours come in view, the mercury begins to fall in the Barometer; but yet these vapours do not contribute any thing to it's descent:  When or before the vapours and clouds begin to disappear in our curcum=ambient air, the mercury begins to ascend; and yet that serenity of the air is no more the cause of this ascent of the mercury, than the ascent of the mercury is the cause of that serenity.  It theorefore is a [[underline]] fallacy [[end underline]] to refer one of those [[underline]] phanomena [[end underline]] , which happen about the same time, to the other as the cause, which is done by most persons.
     But to make this still more apparent.  Set a glafs cylinder, 3 or 4 inches diameter, open at both ends, and of a sufficient length to receive a portable a Barometer, upon the orb of an air pump, covered with water and wet leather, that watery vapours may enter the contained air; when replete, put in a Barometer, closed the top exactly, and exhaust the air.  as the procefs goes on, a [[underline]] mist [[end underline]] will begin to [[underline]] rise [[end underline]] in the cylinder, and the mercury also in the barometer to subside at the same time: both indeed from one and the same cause, but neither the cause of the other.  Admit the air again into the glafs cylinder, and both the [[underline]] serenity [[end underline]] of the included air will immediately ensue, and the mercury in the barometer will ascend at the same time or a little before; and yet it is evident that neither depends on the other; nay, it is manifest that it is not rendered [[underline]] heavier [[end underline]] in one case or [[underline]] lighter [[end underline]] in the other.  Thus art in some measure imitates nature; but because of the wonderful complication of natural causes it cannot be complete, if sufficiently exact.
     Hence a commonly received opinion and a great obstacle is removed, yet the whole businefs is not brought to a conclusion.  For it still remains to know by what causes these alterations of the air are produced.  "The causes which shew how easily "the air is expanded and varified" by any approaching "heat, and particularly how great is the force of the "[[underline]] sun-beams [[end underline]] falling perpenducularly, will shew, or "perhaps this alone will seem sufficient, how the "equilibrium of the air is taken away, if there were

[[Side note reads:]]  Causes of the Alterations of the weight or prefsure of the air to be still considered.