The Weight of the Atmosphere in England does not exceed 311⁄2 Inches of Mercury in the Barometer: nor does the least Weight fall short of 281⁄2: the greatest Difference in the Weights may be taken at 2 Inches: dividing 30 (nearly equal to the whole Weight) by 2, the Answer is 15. So that the under Parts of the Atmosphere being pressed upon by about a fifteenth Part less Weight at one Time, than at another; the specific Gravity of the Air will sometimes be a fifteenth Part lighter.
But the Height of the Atmosphere being estimated at 45 Miles, which is equipoised by about 30 Inches; when equipoised by a fifteenth Part less Weight; (that is, dividing 45 Miles by 15; which amounts to the same as if a fifteenth Part of the whole Height was taken away; the Answer is 3 Miles;) shews that the Atmosphere is 3 Miles higher at one Time than at another, over certain Places; indicated by the Barometer at those Places.
Such an Accumulation of Air, arising only from Pressure or specific Gravity in one Part of the Atmosphere, and not in another; by its Tendency to an Equilibrium; and when to this Tendency is added its elastic Force;—must be productive of winds, descending Torrents, Inundations of Air, or Storms, near the Surface of the Earth: and nearly such a Difference in the Barometer has been known to happen in a few Hours.
Such Accumulation, however, is not properly the Tide of Air.
291. At the New and Full Moon, the united Attractions of the Moon and Sun raise the Spring Tides in the Ocean to the average Height of 10 Feet and a half.[93]
And in the Moon’s Quarters, the Moon drawing one Way, while the Sun draws another, viz. at a right Angle, made by Lines from the Sun and Moon to the Earth’s Center; the average Height of the Neap Tides in the Ocean will be 6 Feet 7 Inches.
The same Attraction which raises Water 10 Feet and a half, will raise Air, whose Density is 800 Times less, to almost one third of that to which the whole Pressure of the Atmosphere can raise Fluids:[94] Now it has been before seen, that the Pressure of the Atmosphere raised the Air 45 Miles: so that the Air is raised by the united Actions of the Moon and Sun, at the New and Full Moon, to one-third Part of 45; i. e. to 15 Miles. And for the same Reason, the Air is raised at the Moon’s Quarters to 10 Miles:[95] the Difference between which is 5 Miles.
There is consequently a real Tide of Air five Miles higher at each New and Full Moon, than at her Quarters: which Tide rolls with incredible Velocity along the Verge or highest Limit of the Atmosphere; and is generally productive of Wind below.
292. The Elasticity of the Air must likewise be brought into the Account, as contributing greatly to its Motion: the Spring of Air always increasing as the Pressure encreases.
Considerable Changes must therefore ensue in the inferior Parts of the Atmosphere.