3. That upon very great Winds, though they be not accompanied with Rain, the Mercury sinks lowest of all, with relation to the Point of the Compass the Wind blows upon.
4. That cæteris paribus the greatest heights of the Mercury are found upon Easterly and North-Easterly Winds.
5. That in calm frosty Weather the Mercury generally stands high.
6. That after very great Storms of Wind, when the Quicksilver has been low, it generally rises again very fast.
7. That the more Northerly places have greater Alterations of the Baroscope, than the more Southerly.
8. That within the Tropicks and near them, those Accounts I have had from others, and my own Observation at St. Helena, make very little or no Variation of the height of the Mercury in all Weathers.
Now that Theory that can well account for all these appearances, will, in all probability, approach nearer the true cause of the Barometers Variations, than any thing hitherto afforded; and such an one I am bound to believe, is that which I here lay down with submission to better Judgments.
I conceive that the principal Cause of the rise and fall of the Mercury, is from the variable Winds, which are found in the Temperate Zones, and whose great unconstancy here in England is most notorious. I shall not at present inquire into the Cause of its uncertainty, but the Matter of Fact being most undoubted, the Legitimate Consequences thereof must be allow'd me, let it proceed from what it will.
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.