On the prefeiit State of A'iroJIatmu 345 



outfide is found preferable to the former method of glvino^ an 

 internal as well as an external coat. The compolitions for 

 varnifhintj balloons * have been varioufiv modified ; but, 

 upon the whole, the moll approved appears to be the bird- 

 lime varnifli of M. Faujas St. Fond, prepared after Mr. Ca- 

 vallo's method as follows : 



*' Inorder torender linfeed oil drying, boil it, with two ounces 

 of fugar of lead and three ounces of litharge \ for every pint 

 of oil, till they arc dillblved, which mav be in half an hour. 

 Then put a pound of bird-lime and half a pint of the drying 

 oil into an iron or copper veflel whofe capacity fhould equal 

 about a gallon, and let it boil very gently over a flow char- 

 coal fire till the bird-lime ceafes to crackle, which will be ia 

 about halt or three quarters of an hour : then pour upon it 

 two pints and a half more of the drying oil, and let it boil 

 about an hour longer, fl:irring it frequently with an iron or 

 wooden fpatula. As the varnilh whilft boiling, and efpe- 

 cially when nearly done, fwells very much, care fliould be 

 taken to remove in thofe cafes the pot from the fire, and re- 

 place it when the varnifli iubfides ; othcrwife it will boil 

 over. WhiHt the fluff is boiling the operator fliould occa- 

 fionally examine whether it has boiled enough ; which may 

 he known by obfervins; x\hether, when rubbed between two 

 knives ar.d then feparated from one another, the varnifli 

 forms threads between them, as it muft then be removed 

 from the fire : when nearly cool, add abovit an equal quantity 

 of fpirit of turpentine : in ufing the varnifli, ihc ituff mufl be 

 ftretched and the varnifli lukewarm : in 24 hours it will be 

 dry." 



Every balloon fliouId be provided with a valve fo con- 

 ftrufted as to open inwards, having alio a fpring to keep it 

 ftuit. The firing by which this valve is regulated mufl; pafs 

 through a fmall hole in the lower part, of the halloon to the 

 car; its lenglh mull be fuch as to allow feveral coils of it to 

 remain in the boat : this is a requifite precaution, as, when 

 ihe inflammable air is much expanded, the horizontal dia- 

 meter of the balloon is fo nmch more extended, and its lower 



• As the tlaftic gum, known by the name of Indian rubly;r, has been 

 much extolled ris a varnifii, the following method of ni\kin!.'; it, as prac- 

 tifcd by Mr. Hl.mchajd, may nut prove unacceptable: — Diirilve clal>ic 

 gum, cut ftn <11, in live times its wtiijlit of rtftilied cflTcnti.il oil of turpen- 

 tine, ^ethereal Ipirit of tuiptntine of the (hops,) by kce|)ing them fonicdayj 

 tof^etherj then boil one ounce of this folution in eight ounces of drying 

 linfeed oil for a few minutes ; flrain the folution, an<i ufc it warm, 



1- The metallic oxides arc lefs exceptionable in the outer vaniifti of » 

 bdlliKin than iiitein;diy, asdicliift fo:iking in chyinj; oil lervvs 10 l]licl4 

 cite external Viiinilli fi^jui tiic ai::tiun uf the ^as. 



pole 



