A E R 



A E R 



eofifcquently— divided by |., gives 265. feet, which is the 



diameter of a Montgolfier that wjll jufl float : but if inflam- 

 mable air, t the weight of common air, be ufcd, the diffe- 

 rence between li and ~ of it is one ; by which d'viding 



~ or 10 -J, the quotient loj feet will be the diameter of an 



inflammable air-balloon that will juft float. If the diameter, 

 in either of thefe cafes be increafcd, the refpeftive balloons 

 will afcend in the atmofphere. 



In order to determine the height to which a given bal- 

 loon will rife, when the diameter of the balloon, and the 

 weight that exactly balances it are given, proceed in the 

 following m.anner : — compute the contents of the globe in 

 cubic feet, and divide its rellraiuing weight in ounces by 

 this content, and the quotient will be the difference in denfity 

 or fpecific gravity of the atmofphere at the furface of the 

 earth, and that at the height to whi' h the balloon will rife ; 

 fubtraft this difference or quotient from li or I, 2, the 

 dcnfity at the earth, and the remainder will be the denfity 

 at that height ; then the height correfponding to that dcn- 

 fity will be found with fufhcient exaftnefs in the annexed 

 Table. 



e. <^. Let the jiiameter of the bal- 

 loon be 35 feet, its capacity 22458, 

 and the levity of the firit 976 pounds, 

 or 15616 ounces; the quotient of 

 the latter number divided by the for- 

 mer, /. e. — ; — is .695, which is the 



and 



224-5 

 denfity at the utmoft height, 

 to which in the Table correfponds 

 fomov/hat lefs than 2 i miles, and 

 this is the height to which the 

 balloon will afcend. \Vlien the 

 fame baUoon was filled with heated 

 air, its levity was equal to 133 

 pounds, or ZI28 ounces, which di- 

 vided by 2245S, the capacity, gives 

 the quotient, .095 ; and this iub- 

 tracled from i.200, leaves 1. 105 for the denfity; to which, 

 in the Table correfponds half a mile, or more nearly -|- of 

 a mile. Such are the heights to which thele balloons 

 would nearly afcend, if they retained their figure, and loft 

 none of the contained air : or, more precifely, thefe are 

 the heights at which they would fettle ; for their acquired 

 velocity would at firll carry them above thefe heights, till 

 their motion would be deffroyed ; and then they would 

 defeend below thefe heights, though not fo much ap they 

 had gone above them : after v.'hich they would reafcend, 

 and pafs thefe heights again, but not fo far as they had 

 gone below them ; thus vibrating alternately above and 

 below thefe heights, but every-time lefs and lefs. Thefe 

 calculations for finding the height to which the balloon will 

 afcend, are formed independently of the different ftates of 

 the thermometer at the highcft point and at the furface of 

 the earth ; but the allowances to bs made on this account 

 will appear from what is delivered under the article At- 



M0SPHER.E. 



Next to the ihape, it is neceffary to confider the fluff that 

 is moll proper for forming the envelope of the inflammable 

 0( rarefied air. Silk fluff, efpecially that which is called 

 liUellring, properly varnifhed, has been moil commonly 

 ufcd for inflammable air-balloons : and comm.on li;ien, lined 

 within and without with paper, varnifliod, for thofe of 

 rarefied air, Vaniilhed paper, or gold beater's flcin, will 

 aufwer the purpole for making fraall inflammable air-bal- 



loons ; and the fmall rarefied aii-b:il!oon3 may be made of 

 paper without any varnilh or other preparation. 



The fluff for large balloons of both kinds require fomc 

 previous preparation. The bell mode of preparing the 

 cloth for a machine upon Montgolficr'3 principle, is firfl to 

 foak it in a foluti'^n of fal ammoniac and fiic, ufing one 

 pound of each to everj' gallon of water ; and when the 

 cloth is quite dr)', to paint it over with fome earthy ciJour, 

 and flrong fizc or glue. It may be alfo varnlfued over, 

 when pcrfeftly dry, with fome lliif oily varnifli or finiple 

 drying linfeed oil ; which would dry before it penetrate* 

 quite through the cloth. 



The varnifh for the filk or linen of the inflammable air- 

 balloons fhould be impermeable to the inflammable gas, 

 pliable, and fufficicntly dry to adhere firmly to the ftufi. 

 In Fiance much has been faid of the elaftic gum varnifli ; 

 but the compofition of it is kept a fecret. This gum i« 

 known to be foluble in divers effential oils, and a'fo by 

 vitriolic ether. The former folution fonns a varnifli which 

 never perfectly dries : the latter dries readily, but the folu- 

 tion is too dear for common ufe. The following varnifli 

 has been recommended. To one pint of linfeed oil, add 

 two ounces of litharge, two ounces of white vitriol, and 

 two ounces of gum fandarach ; boil the whole for about aa 

 hour over a flow fire ; then let it cool : feparate it from the 

 fediment, or ilrain it through a fieve, and dilute it with a 

 fufficient quantity of fpirits of turpentine. But the befl 

 varnifli for an inflammable air-balloon is made with bird-hrae. 

 Mr. Cavallo direfts to prepare it in the following manner, 

 which, iu his opinion, is preferable to that of M. Faujas dc 

 Saint Fond. In order to render linfeed oil drying, boil 

 it with two ounces of facchariim faturni and three ounces of 

 litharge, for every pint of oil, till the oil hath diffolved them; 

 then put a pound of bird-hme and h;df a pint of the dr)-ing 

 oil into a pot of iron or copper, holding about a gallon ; and 

 let it boil gently over a flow charcoal fire till the bird-hme 

 ceafes to crackle ; then pour upon it two pints and a half of 

 drying oil, and boil it for about an hour longer, flirring it 

 often with an iron or wooden fpatula. As the varnifli in 

 boiling fwelis m,ueh, the pot fhould be removed from the 

 fire and replaced when the varnifh fubfidc;. Whilfl it is 

 boiling, it fliould be occafionally examined, in order to "de- 

 termine whether it has boiled enough. For this purpose, 

 take fome of it upon the blade of a large knife, and after 

 rubbing the blade of another knife upon it, feparate the 

 knives, and when on their feparation the varnifli begins to 

 form threads between the two knives, it has boiled enough, 

 and fhould be removed from the fire. When it is ahnofl 

 cold, add about an equal quantity of fpirits of tui-pciitine, 

 mix both well together, and let the mafs reft till the next 

 day ; then having warmed it a httle, ftrain and bottle it. 

 If it is too thick, add more fpirits of turpentine. This 

 varnifh fhould be laid upon the ftufi", when perfecftly dry, 

 in a luke-warm ftate ; a thin coat of it upon one fide, and 

 about twelve hours after two other coats fliould be laid on, 

 one on each fide, and in twenty-four hours the filk may be 

 ufcd. 



Mr, Blanchard's method of making elaftic gum varnifli for 

 the filk of a balloon is as follows, Diffolve elaftic gum, cut 

 fmall, in five times its weight of fpirits of turpentine, by 

 keeping them fome days together ; then boil one ounce of 

 this fohition in eight ounces of drying linfeed oil for a few 

 minutes, and ftrain it, Ufe it warm. 



The pieces of which an inflammable air-balloon is to b* 



formed, muft be cut of a proper fizc, according to the pro- 



poftd dimerfions of it, when the varnifli is fufficiently dry. 



The pieces that compofe the furface of the ballooa are like 



0^1 2 thafe 



