The composition of the air, at the height of 21,849 feet, has been found to be the same as at the surface of the earth. Various currents prevail at different alti tudes, and some of them in directions diametrically op to each other. Storms, too, are frequently local and, when the aeronaut surmounts the region of their influence, he is safe. The velocity with which a bal loon is transported in the fury of a tempest, if not so well authenticated, would almost stagger our belief; in stances have occurred where it has not been less than 70 or 80 miles an hour. The structure of the clouds has, in appearance, been compared by some to a plain of snow ; and by others, to a sea of cotton. Some, again, have compared those charged with electricity to the smoke of ordnance. Clouds consisting of hail or snow are frequently met with, in such abundance, that these substances may be gathered in handfuls from the car. 0.hers consist of mist or rain, in which the aeronaut, who is every moment exposed to a change of tempera ture, is copiously drenched. Notwithstanding all the changes experienced in an aerial voyage, the gas con tamed in the balloon always preserves a much .higher degree of heat than that of the surrounding atmosphere ; a fact that has not as vet been satisfactorily accounted for. Birds, or other winged animals, when allowed to escape at a great height, either fall down with precipi tation, on finding the quality of the fluid different from what they have been accustomed to, or they descend obliquely in their flight, describing long curves similar to those cf birds of prey.
C9nAt•uction rf Balloons.
In the formation of balloons, three things are to be at tended to : the structure, of the balloon containing the air w hien produce, it, ascent ; that of the parachute ; and of the car or boat, which receives the aeronaut. Balloons are either spherical or elliptical ; the spherical halm however, has been almost 1111i1crsally adopted, probably because a sphere admits the greatest capacity under the smallest surface. Balloons filled with rare fied air, have usually been made of common linen, soaked in a solution of alum to obviate the risk of fire, and varnished to prevent the escape of the air. It is of advantage to have them of a considerable size, because a smaller quantity of fire will produce a greater propor tional rarefaction, and it is besides attended with less danger. It has been thought that the most eligible figure for a balloon with rarefied air, is an inverted cone, or a truncated pyramid, as it would allow the heated air, which has both a tendency to ascend and to expand, to occupy the wide part above, while the lower part would contain the colder air. Experiments, however, have proved, that the ascensive power depends by no means on the figure of the balloon. Mr Cavallo recom mends, that the opening of a rarefied air balloon above the fire, should be one-third of the diameter of the bal loon itself, if the size exceeds 50 feet ; and that it should project from the balloon by a cylindrical neck. The
gallery for the aeronauts is placed on the outside of this neck, and the fire-place for rarefying the air within it. Above the edge of the gallery, holes are cut for introducing fuel to the fire. Small balloons after this method, may be made of paper, with a wire round the bottom. Two or three cross wires are fixed in the centre to support a cup, containing cotton and spirits of wine, the flame of which rarefies the air, and produces the ascent. We shall now proceed to consider the con struction of balloons filled with inflammable air, whose superior advantages deserve a more detailed and atten tive description.
The substance uniformly used for the envelope of in flammable air, is silk lusting, which, from its close texture, strength, and lightness, is peculiarly suitable fur the purpose. The price, however, of this material is so extremely high in Great Britain, as to render the con struction of a balloon even of a middling size very ex pensive. Hence a late judicious writer on this subject suggests the expedient of substituting strong cambric muslin, rinsed in drying oil, previously to the junction of the separate pieces. In calculating the weight and quantity of cloth requisite for constructing a balloon of a given diameter, we have only to multiply the square of the diameter by 3.1416, and the product will be an area of the surface of the sphere, or the quantity of cloth ne cessary for its formation. Thus, if the balloon be 12 feet diameter, we have 12 x 12 X 3.1416=452 square feet nearly, or 50 square yards for the quantity of cloth. The weight of this quantity of cloth will be found by multi plying the number of square yards by the weight of one yard of the cloth. The solid contents of the balloon may be found by multiply ing the cube of its diameter by 0.5236.
When the capacity of the balloon is determined, it will not be difficult to ascertain its power of ascent. A cubic foot of atmospherical air weighs about 1.2 oz. ; whence a quantity of air, equal in bulk to the solid con tents of a spherical balloon, 35 feet in diameter, weighs 26,950 ounces, or about 1684 pounds. Suppose the in flammable gas in the balloon is six times lighter than common air, then the weight of it is 280 pounds, which occupy the same space a the atmospheric air displaced ; n this add 423 pounds, the weight of the bag or envelope, and the w hole makes 708 pounds : deducting this num ber ft om 1634, the weight of common air displaced, there remain 076 pounds, as the ascensive power, or specific levity of the balloon. This method of computation will lead to the proper results, whatever be the dimensions of the balloon, or the specific levity of the gas. Thus, a a balloon 30 feet in diameter requires 314 yards of cloth, and its ascensive power will be 581 pounds. One, 20 feet in diameter, requires 140 yards, and its power is 122 pounds.