315. 6. Different Trials may be repeatedly made: the Effects of which, whether evidently useful or _apparently otherwise_, being carefully recorded and regularly published _in Detail_, may afford Data for the Prosecution of further Discoveries, and lay the Foundation for a rational Superstructure of _airostatic Navigation_. [Sidenote: On the Manner in which the Wind, Anemòmeter, and propulsive Machinery will probably operate on the Balloon.] Sect. 316. Art. 1. By adding Weights, and encreasing the Surface of Anemòmeter-Sails; the Vis Inertiæ will become so powerful in the Direction of the resisting Medium of the Air; that the Wind in the opposite Direction will force the Balloon out of its Vertical, and incline it to the Horizon. The Car will be a Fulcrum Axis or Center of Motion: on an imaginary Point of which, as on a Pivot, the Balloon and Sails will turn opposite Ways, balancing each other in every Situation. 316. 2. The Balloon must therefore be brought back into the Vertical by a counter Exertion of the Wings: to which the Vis Inertiæ must always be made to bear a just Proportion. The Declination of the Balloon is the only Inconvenience foreseen to result from an Anemòmeter too large, or too heavily laden: and it is instantly remedied by slacking the Sail. One Thing still remains to be mentioned. 317. Balloons _durably_ Air-tight, and terminating in a _Hemisphere_ above, (Section 307); ought to have their Dimensions such, that there shoud be no Occasion for more than their upper Hemisphere to be inflated. Under which Form, they may with Ease and Safety be pitched as Tents on the Ground; by Cords fastened at equal Distances to the equatorial Hoop; and on Occasion by the Aironaut himself, while in the Car: who may be provided with Iron Ring Stakes barbed, and fastened or ready to be fastened to each Balloon-Cord: and, as soon as the Balloon is moored by the Anchor, Grapple, and snatch Block, (Section 298, 3) with a light Axe drive down the Stakes round the Car, and regulate them when he alights from it, on the Ground. CHAPTER LXI. HINT FOR A VANE-SAIL TO PREVENT THE BALLOON FROM TURNING ROUND, WHILE THE WIND CONTINUES STEADY. [Sidenote: Hint for a _Vane-Sail_.] Section 318. To the Block-Pulley in the equatorial Hoop, hoist a Sail, whose Shape is as follows. From the equatorial Hoop, let fall a Perpendicular: and from the lowest circular Point in the Circumference of the Balloon, draw a Tangent, or horizontal Line, till it meet the former: these Lines, together with that Part of the Circumference intercepted between them, in the Points where they touch the Circle, forms a Space, which is the Shape sought. The Sail may be kept steady by a hollow Cane or Bowsprit thrust out from the Car, and made fast with the usual Tackling.