ln sewing together the corresponding edges of the separate pieces of the envelope, one is left half an inch broader than the other, and is turned over in a subse quent row of stitches. This double seam is put be tween two pieces of brown paper, and pressed with a fire-iron, heated just enough to soften the drying oil. One or two tubes of the same material as the balloon. are used for introducing the gas, and these are tied up whenever it is filled. A balloon of 30 feet diameter re quit es two tubes of six inches. The whole external sur face is covered with a coat of varnish. The following composition, prescribed by Cavallo, is the best for this purpose. "Boil a pint of linseed-oil with two ounces of sugar-of-lead, and three of litharge ; and when they are dissolved, put a pound of birdlime, and half a pint of the drying oil, into an iron or copper vessel, and allow it to boil gently over a slow charcoal fire, till the birdlime ceases to crackle, which may require about half an hour or three quarters. Then pour two pints and a half more of the drying oil upon it, and let it boil about an hour longer, taking care to stir it frequently. It will be found to be sufficiently boiled, when, by rubbing a little of it between two knives, the varnish, on their separation, draws into threads. After it is removed from the fire, and nearly cold, add about an equal quantity of spirit of turpentine." When the varnish is to be used, it should be lukewarm, and the balloon stretched out. It dries in the space of 24 hours. Another varnish has been made from a pre paration of caoutchouc, or Indian rubber. This is done by dissolving that substance, cut into very small pieces, in five times its weight of ethereal spirit of turpentine, which is effected by keeping them for some days to gether ; then boil an ounce of this solution a few minutes in eight ounces of drying linseed oil ; and having strained it, apply it while warm to the balloon.
As the balloon would be in danger of bursting, by the degree of expansion which the included air undergoes in the higher regions of the atmosphere, a valve is em ployed for allowing the air to e.,cape, when the balloon is too much distended. It has generally been placed at the top of the balloon, though the equator of the balloon, has been recommended as the best position for the valve.
The car, whose size and figure are quite arbitrary, is made of wicker, covered with leather, and well varnished. It is attached to the balloon by cords, proceeding from a net-work, which covers the upper half of the balloon. These cords are collected about two feet below the balloon, in the circumference of a circle, from which other cords are distributed down to the edges of the car. The meshes of the net-week are smaller above, because it is there that the greatest strength is required. la some instances, it is connected by moans of a hoop, made usually of wood or cane, and encircling the balloon. The car is represented in Plate III. Fig. 3., and also in Plate IV, Fig. I. at CD.
Th.. exact dimensions which a parachute ought to have, in order to be an instalment of perfect security and confidence, have not yet been ascertained. The para chute, by which Garnerin descended at Paris in 1707, was 25 feet in diaieter, and was made of cloth ; and that by which he descended at London in t8u2, is said to have been a large umbrella, consisting of 32 gores of canvas, 23 feet in diameter, and without ribs and handle. At the top there was a round pie( c of wood, 10 inches in diameter, having a hole in the centre, which was fast ened to the canvas by S2 short pieces of tape. About four feet and a half front the top of the canvas, a wooden hoop, eight feet wide, was put on and tied by a string from each seam Several ropes, about 30 feet long, pro ceeding from the edge of the parachute, terminated in a common joining. From this point there issued short er ropes, to whose extremities was fastened a circular basket, in which Garnerin himself was stationed. The parachute and basket were immediately disunited front the balloon, by the cutting of a cord •h?ch com municated with the net-work, and in falling downwards, the parachute naturally expanded, by the resistance of the air. Several improvements on the parachute have been suggested; viz. by constructing it of varnished cambric muslin, or light linen, covered externally with a netting firmly fixed to it ; by making the diame ter, when expanded, not more than 20 feet, and forming hut a small segment of a sphere ; by having a central aperture in the segment, fully three feet in diameter, to allow the escape of the air as it descends ; and by fur nishing this with a shutter on a hinge, which the aero naut might close or open, according to circumstances, by means of a cord connecting the shutter with the bas ket. The parachute, when open, is represented in Plate IV. figure 2 ; and figure 3 represents it when shut.
Since it would be a work of infinite labour to collect hydrogen gas as it exists in its original and natural state, the method of procuring it from the chemical ac tion of bodies is always resorted to as the most conve nient and productive. It may sometimes be obtained in
such purity, as to be twelve times lighter than common air, but in general its specific levity is only live or six times greater. It is usually procured from a solution of iron or zinc in sulphuric acid. The iron best adapt ed for the purpose, is the turnings produced by the bo ring of cannon ; but when this cannot be obtained, chips of iron should be preferred to filings. It is of impor tance to attend to the purity of the metal ; for rust produces hydrocarbonate, a gas which is specifically heavier than atmospherical air. Grease also is injuri ous, because it resists the action of the acid. The sul phuric acid must be diluted with five or six times its weight of water. Iron yields about 1700 times its own bulk of gas ; therefore, 41 ounces of iron, with the same weight of sulphuric acid, and 224 of water, will produce a cubic foot of inflammable air ; and of zinc, 6 ounces, with the same quantity of acid, and So ounces of water, will produce a cubic foot of air. The gas is collected into a number of casks, which are disposed round a large cask or cooler ; the cooler contains water mixed with unslacked lime, and receives the gas from the su• rounding casks through tin tubes, which connect them with each other ; and the gas, purified from any carbo nic acid gas, which may have incorporated with it be fore it passed through the water, is conveyed by a silken tube from the cooler into the balloon. The smaller casks in which the solution is carried on, ought to be lined with tin. NI. Garnerin, in 1802, used 36 casks, every 12 of which communicated with a cooler, and three. tubes from the three coolers conveyed the gas into a large tube, which was joined to the balloon. Profes sors Robertson and Sacharoff had 23 vessels communica ting with a cooler, into each of which they put 120 pounds of iron filings, (chiefly from cast-iron,) along with 600 pounds of water, and 129 pounds of sulphuric acid pour ed over it. The filling of the balloon occupied live hours. M. Blanchard tilled a balloon 21 feet in diame ter, from only four casks, each holding 120 gallons. Lu nardi, a celebrated aeronaut, reducing- the apparatus to still greater simplicity, employed only two casks, from which the gas was transmitted into the balloon without passing through water ; and in the short space of half an hour, he filled the balloon in which he ascended from Edinburgh and Glasgow ; it was of a pcar-shape, 23 feet in diameter, and :30 in height. Blanchard used 1000 pounds of iron, and 1250 of sulphuric acid, for producing the gas to fill a balloon of 21 feet. Lunardi, on the occa sion we have just now mentioned, employed 2000 pounds of each, and 12,000 of water. The latest writer on this subject computes, that this quantity should suffice for a balloon of 30 feet in diameter, which is 14,137 in capaci ty. The balloon of 33 feet, in which Lunardi first as cended in England, and one that ascended from Nantz about the same time, were filled from zinc instead of iron. Making allowance for the expansion of the gas during the ascent, the balloon is never filled above three fourths. There is also a method of procuring hydrogen gas, by passing water over metals, or through tubes pre viously heated to redness. Although there is a danger of the metal running to a slag b.2fore any considerable quantity of gas is obtained, the aeronautic institution at Mention, erected an apparatus, consisting of a series of red-hot cylinders, from which they filled their balloon, 32 feet in diameter, in the space of eight hours. This method is found sufficiently productive, and is more eco nomical than the former.
In filling balloons, great care should be taken that they be fixed in such a manner as to prevent them from escaping prematurely into the air. Instances have hap pened, in which a disregard of this precaution has proved fatal to persons who were holding the balloon by the balloon, on acquiring a sufficient ascensive power, suddenly rose into the air, before those who were at tempting to restrain it could extricate themselves from their hold. The exercising balloon at NIendon had an ascensive power of 800 pounds, which is equal to the strength of 20 people, and it was retained by means of a capstan.
We shall conclude this subject by observing, that sails and oars, which have both received the proof of experi ment, have been found of little or no advantage in guid ing balloons. Wings or oars have indeed in some rare instances produced an impression on the balloon's course which was barely sensible, and have therefore received no sanction from general use. The total inability of the aeronaut to impress on the balloon a determinate direc tion, is the greatest defect in the science of aeronautics.
Al. Charles and Roberts' balloon. Sec p.
160. col. 1.
Fig. 5. Pattern for cutting the pieces or gores to form a balloon. These gores arc constructed in the same manner as those 01 globes. On figure 5. are engraved the breadths of the gore at different places, the greatest breadth being 1.000.