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. For as the Effect of the Moon’s Attraction is to diminish the Weight of the Atmosphere (tho’ its Quantity be increased) by elevating the Column of Air in the Line of her Meridian; the Rarefaction of the Air is therefore encreased, first _at the Top_ of the Atmosphere; afterwards it gradually descends to the Bottom, or Surface of the Earth: so that the incumbent Weight being diminished, the Air beneath will be greatly _expanded_. At whatever Height therefore any _Quantity of Vapour_ or superior Cloud _rested_, while the Moon was in her Quarter; it woud _gradually descend_ at the Approach of the next New or Full: at which Times it woud remain suspended at a Height, where an Expansion took Place equivalent to the former Expansion, at the Moon’s Quarter: and, if the Height during the Moon’s Quarter was only equal to that of common Clouds; such Vapour woud, at the New and Full Moon, _descend_ in Mist, Rain, Snow, or Wind. 293. Little Reliance is to be placed, in these _Northern_ Climates, on the aggregate Weight (_or elastic Power_) of the Air, indicated by the Height of the Barometer, near the Times of the New and Full Moons: tho’, in general, it will _descend_ about those Times. [Sidenote: Proper Days for Ascent.] These Things being so; it woud be improvident to undertake an aërial Excursion, either three Days before, or three Days after the Day, either of the New, or Full Moon: the Ascent shoud be forborne every other Week; at least till the Art is a little more advanced.