344 On the prefcnt State of AeroJIatton. 



the price of this filk, though more moderate in P'rance, is in 

 this country fo high * as to augment the expenfe of amiddling- 

 fized balloon ahiioft to a prohibition to any but the wealthieft 

 individuals. I do not fee, therefore, why ftrong cambric 

 muflin, rinfcd in drying oilf, (previous to fewing the pieces 

 together,) fliould not fully anfwer every purpofe. After pro- 

 viding the nccelVary quantity of ihe (UifT, and each piece hav- 

 ing been properly prepared with the drying oil, let the cor- 

 refponding edges be fewed together in fuch a manner as to 

 leave about half an inch of one piece beyond the edge of the 

 other, in order that this may, in a fubfcquent row of flitches, 

 be turned over the latter, and both again fewed down toge- 

 ther : by fo doing, a confulerable degree of ftrength is given 

 to the whole bng at the feams, and the hazard of the gas 

 efcaping doubly prevented. Having gone in this manner 

 through all the icanis, the following method of Mr. Blan- 

 chard is admirably calculated to render them yet more per- 

 fe6lly air-tight. The feam being doubly ftilched as above, 

 lay beneath it a piece of brown paper, and alfo another piece 

 over it on the outfide; upon this latter pafs feveral times a 

 common fire-iron healed juft fufficicntly to foften the drying 

 oil in the feam : this done, every interftice will be now clofed, 

 and the feam rendered completely air-tight. 



The neck of the balloon being left a foot in diameter and 

 three in length, and all the feams finiflied, the bag will be 

 ready to receive thevarnifhj a fingle coating of which on the 



in a foJution of alum or fal-ammoniac and common fize : Mr. Ca- 

 vallo recommends a gallon of water to a pound of each of the two ingre- 

 dients: it may then be fewed into the required (hape, and afterwards once 

 varniflied on the outfide with merely drving linfeed oil, adding a fmall 

 quantity of neat's-foot oil to prevent its being fticky. The liquor fiUcuviy 

 cr liqnor of flints of the chemifts, prepared by melting together one part 

 of fand or powdered flints and four parts of fixed alkali, might, perhaps, 

 be advantageoufly fubftituted for the faline ingredients above mentioned, 

 as rendering paper or cloth varniflied therewith pcrfe£lly mcombuftiblc. 



" Silk luftring may be purchafcd on the continent at five fliillings the 

 yard, but here for not Icfs tlian nine. 



f In rendering oil drying for ai-roftatic purpofes we fliould avoid the 

 ufe of nittallic oxides, as litharge, &c., which rot the liiiff of the enve- 

 lope; befides that hydrogen gas, by gradually reducing the metals, cor- 

 rodes fuch vamiih, turning it black, and in time caufing it to crack. The 

 btfl: method to prevent thefe inconveniences is to allow the oil to (land for 

 leveral weeks ovc unflaktd lime, or to diflblve in it a fmall quantity of 

 gum-fandarac, gum-lac, maftich, feed-lac, or common refin ; all of which, 

 when finely powdered, may be diflblved in linfeed oil by well boiling, 

 thu» communicating to the oil the property of drying, yet retaining clafti- 

 city. Silk and canvas for umbrellas, &c. are varniitied with a folution of 

 cither refm or gum-lac, melted with drying linfeed oil to the confidence of 

 a thick balfam, fo as not to run about : thus varniflied they are very little ■ 

 heavier than before, though impermeable to air or water, 



outfide 



\ 



