CHAPTER LII.

[Sidenote: _Proper_ Days in the Month for the Ascent of Balloons.]

Section 285. As the _safest Hour_ of the Day has been already pointed out, for the Ascent of those Aironauts, who propose to cross a Channel, or Arm of the Sea, in a Balloon _Air-tight_ or nearly so: it may not be useless to throw out a few Hints on the properest Days in _each Month_, for the Ascent of Balloons.

286. It will perhaps be found true, that the more frequent Winds are generated near the Surface of the Earth: but that _Storms_ are generated from above. Cold, Heat, Drought, and Moisture produce the more frequent and diurnal Winds: but the Conjunctions and Operations of the Moon and Planets contribute to the Production of Storms and other Inequalities of the Atmosphere: more especially the _Moon_: at the New and Full. These Attractions first affect the _superior Parts_ of the Atmosphere.[89]

287. “We are sure in the calmest Weather, to have some Breeze at Noon, and at full Tide.” Therefore, both are improper Times for Balloons to be at Sea: the Time of low Water and Midnight woud be best in those, if equal in other Respects.

Changes of Weather as to Wind or Calm happen about the New and Full Moon.[90]

288. Varieties of Tide produced by the united or divided Forces of the Sun and Moon, occasion similar Changes in the Atmosphere nearly at the same Time.

For Instance, at the Time of the New Moon or Conjunction, i.e. when the Earth, Moon, and Sun, are _nearly_ in a Line; the Moon being between them: also at the Time of the Full Moon; i.e. when the Moon, Earth, and Sun are _nearly_ in a Line; and the Earth between them, which is called the Opposition.[91]

In the first Case, the Moon and Sun attract the Atmosphere of the Earth conjointly, or with united Force: in the second Case; the Earth being between them, they act in Opposition to each other, still nearly in the same Line.

At these Times, the +spring+ _Tides_ are at the _highest_ i.e. once every Fortnight; and in the two interval Weeks are the +neap+ or _lowest_ Tides: for a like Reason.

Because, in the latter Case, a Line supposed to be drawn from the Moon to the Earth, and another from the Earth to the Sun, woud form nearly a right Angle: or in other Words; because the Moon and Sun woud attract the Earth at right Angles to each other, or in a lateral Direction:—the Moon woud draw one Way and the Sun another:—their Forces woud be divided.

Now it is a Fact, that the Ocean is raised considerably twice every twenty-five Hours, by the Attraction of the Moon, when she comes to the Meridian. So that the Surface of the Sea, instead of putting on the Form of a Sphere, or Globe, will be changed into an _oval_ Figure, whose longest Diameter being produced, woud pass throu’ the Moon.

In like Manner a similar Elevation must take Place, as often as the Sun is in the Meridian; either above or below the Horizon.

Moreover, this Elevation is _greatest_ on the New and Full Moon, because the Moon and Sun do then conspire in their Attractions: and _least_ in the Quarters: as they will then draw different Ways; the _Difference_ of their Actions only producing an Effect.

Lastly, the Intumescence will be of a _middle_ Degree, at the Times between the Quarters, and New and Full Moon.

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.

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.

The two remaining alternate Weeks in each Month, viz. when the Moon is in the Quarters, and the Tide of Air flowing throu’ the Atmosphere, is checked, counterbalanced, and equalized, by the lateral Attractions of the Moon and Sun, acting at right Angles, i.e. on different Parts of the Air, pendent on the Earth’s Surface;—more settled and regular Weather may be naturally expected; and particularly freer from the Extremes of _Wind_ and _Cold_.

Moreover, as the Almanack, and Ephèmeris[96] may be always consulted; the Day fixed on shoud not be _marked_ with Conjunctions of the Planets.[97] The Inequality of their united Attractions greatly deranges the Equilibrium of the upper Parts of the Atmosphere; producing sudden Squalls and Gusts of Wind: which, tho’ of short Continuance, perhaps a few Hours, are inauspicious to the successful Inflation and Ascent of a Balloon, during the Infancy of the Science. (See Section 211.)