A second Cause is the uncertain Exhalation and Præcipitation of the Vapours lodging in the Air, whereby it comes to be at one time much more crowded than at another, and consequently heavier; but this latter in a great measure depends upon the former. Now from these Principles I shall endeavour to explicate the several _Phænomena_ of the _Barometer_, taking them in the same order I laid them down. 1. _Why in calm Weather the Air being inclin'd to Rain, the _Mercury_ is commonly low?_ I Answer, That the _Mercury_'s being low, inclines it to Rain; for the Air being light, the Vapours are no longer supported thereby, being become specifically heavier than the Medium wherein they floated; so that they descend towards the Earth, and in their fall meeting with other aqueous Particles, they incorporate together, and form little drops of Rain; but the _Mercury_'s being at one time lower than another, is the effect of two contrary Winds blowing from the place whence the _Barometer_ stands; whereby the Air of that place is carried both ways from it, and consequently the incumbent Cylinder of Air is diminished, and accordingly the _Mercury_ sinks; as for Instance, if in the _German Ocean_ it should blow a Gale of Westerly Wind, and at the same time an Easterly Wind in the _Irish Sea_; or if in _France_ it should blow a Southerly Wind, and in _Scotland_ a Northern; it must be granted me, that That part of the _Atmosphere_ impendent over _England_, would thereby be exhausted and attenuated, and the _Mercury_ would subside, and the Vapours which before floated in those parts of the _Air_ of equal Gravity with themselves, would sink to the Earth. 2. _Why in serene good settled weather the _Mercury_ is generally high?_ To this I Answer, That the greater height of the _Barometer_, is occasion'd by two contrary Winds blowing towards the place of Observation, whereby the Air of other places is brought thither and accumulated; so that the incumbent Cylinder of Air being encreas'd both in height and weight, the _Mercury_ press'd thereby must needs rise and stand high, as long as the Winds continue so to blow; and then the Air being specifically heavier, the Vapours are better kept suspended, so that they have no inclination to Præcipitate and fall down in Drops, which is the reason of the serene good Weather, which attends the greater heights of the _Mercury_.