But if it be considered that such Air, acting as a wedge, or more probably in the Form of an hyperbòlic Solid,[68] to fill up the Vacuity, descends with Rapidity from a colder Atmosphere impregnated with aqueous Vapours invisible from below; and that both the Air and Vapour have reciprocal Affinities and Attractions, electric and mechanical, with the Body of Water beneath them; and are often rendered still cooler by its constant Agitation and Evaporation; also, that the Supply being immediate and cotemporary, with the double tide of air flowing from the middle over the opposite Shores;—there possibly may be little or no Difference between the aggregate or barometric Gravity of such Columns, and those which are formed by the Sea-Breeze on either Side of them: therefore the Descent of Balloons is owing, among other Causes, to an almost perpendicular actual Depression of the superincumbent Atmosphere.[69]
Following up the Idea of a Sea-Breeze, blowing, at a Medium, for 20 Miles over Land; altho’ the Stratum of the lower current of Air, or Sea Breeze, may not exceed half a mile in depth, measuring from the Ground upwards; nearly equal to 26 Inches of the Barometer above, the Thermometer also above being at 55, i. e. Temperate:—yet this Observation may prove of essential Service, while the upper current of Air, i. e. the general Wind blows towards the Sea, (which will be found to take Place more frequently than is, at present, imagined;) or while the Balloon is influenced that Way; as was the Case with Sadler and his Companion when over the Nore: who, on his accidental and sudden Descent, fortunately found Safety in the sea-breeze.
Which Breeze was sought for, and made Use of by the Author, when in the Balloon, near Frodsham, in Cheshire.
For, as the Sea-Breeze is pretty general, Aironauts shoud not be too apprehensive: as they have it in their Power, by proper Management, to drop into the Breeze—for either shore: if they are provided with a Machinery to waft themselves across the intermediate depressing or accumulating mediocèanal column of air: which Space, between the two Shores, is, as before hinted, frequently becalmed.
258. Further: as the above Theory of a mediocèanal Depression seems to receive additional Confirmation from each Balloon Experiment; Lunardi descending on the 5th of October last, when near the Middle of the Bay of Edinburgh or Firth of Forth;—it may be found prudent, to keep the Balloon continually rising, till the Aironaut is one-third of the Passage over.
258. 2. For if the general Wind in the upper Current be not strong; the Aironaut may expect to be becalmed, with Respect to the horizontal Direction of the Current, the Instant he finds, by the Rise of the Barometer, that the Balloon descends; i. e. when it is acted upon by the depressing Column: in which Case, the higher he has soared, the safer: as he will have more Room and greater Latitude for Exertion by Means of the Machinery: which Machinery will be greatly aided by the Force of the descending Column or Gravity; and will act on a similar Principle with the Ferry-Boats over the River Po in Italy; which are a Sort of horizontal Pendulum. For the Aironauts will continue to descend, at the same Time that their Wings furnish the Means of a progressive Motion.
Therefore, before the Time that the Balloon has reached the Surface of the Water; they will have crossed the depressing Column; and find themselves wafted gently by the new Sea-Breeze setting in towards the opposite Shore.
259. If the Aironaut rises up to Sea with a Wind blowing from the Land on each of the opposite Sides of the Channel, and arrives above the Middle of the Channel, while the same Wind remains; it is probable that the Balloon will continue to rise higher as he proceeds towards the Middle, where the mediocèanal accumulation has for some Hours taken Place; and therefore he need not be under any Apprehension of falling: but, as before, it being probable he will also be becalmed; the Necessity of propulsive Machinery is equally urgent, in order to pass the Center of the Accumulation: after which, the Balloon will ride Home to the opposite Shore in the new Sea-Breeze, by that Time, just beginning to set in.
260. With the Assistance of propulsive Machinery, it is imagined the Aironaut may be enabled in a few Minutes to force throu’ the calm mediocèanal Accumulation, or Depression: after which, he will have little Occasion to make Use of it.
261. Sunrise is, probably, the safest Time of all, to ascend towards the Sea, with an Air-tight Balloon: arriving with the Assistance of the Wings, throu’ the calm mediocèanal Accumulation: and there waiting till the new Sea-Breeze sets in to the opposite Shore.