289. As in the Ocean, so in the Air above it; a Tide of Air must roll along the Atmosphere, throu’ the whole Extent of it; and rise upwards twice in about 24 Hours. And since the Height of the Atmosphere is computed by Halley at 45 Miles, and the Depth of the Ocean at an Average, but half a Mile; the Air will more easily and quickly obey the Attraction of the Moon and Sun, than the Tide of the Ocean: and, as it revolves in a Sphere which is about 100 Times larger than that of the Ocean, the Agitation and the Velocity of its Tide, will be something greater, in Proportion to its Elasticity, and inferior Density to the Water of the Ocean.[92] 290. The _Weight_ of the Air must now be considered. The Weight of the Atmosphere in England does not exceed 31½ Inches of Mercury in the Barometer: nor does the least Weight fall short of 28½: 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.