Chap. 14.3 Ufes of Balloons. 515 



red-hot by burning coals. Dr. Prieftley recommends 

 a tube of red-hot brafs, filled with fmall pieces of 

 iron. By this method inflammable air is produced, 

 the fpecific gravity of which is to that of common 

 air as i to 13. 



The beft varnifh for coating the filk of the bal- 

 loon in order to retain the inflammable air, is that 

 ufed by M. Blanchard, which confifts of elaftic gum, 

 or caoutchouc, cut fmall, and boiled in five times its 

 weight of oil of turpentine, the folution being after- 

 wards boiled for a few minutes with dryingilinfeed 

 oil. This varnifh muft be ufed warm. An aperture, 

 with a valve, to which is attached a cord, muft be left 

 in the top of the balloon, to prevent its buriling, by 

 too great inflation, in the higher regions, where the air 

 is lefs denfe. 



The only practical ufe hitherto difcovered for bal- 

 loons, is that to which the French engineers have ap- 

 plied them in the prefent war, which is, by raifmg them 

 to a convenient height, to enable the engineer to re- 

 connoitre the camp of the enemy, or a fortified place, 

 fo that he can direct the attack to that part which is 

 moft eafily afiailed. 



That fo extraordinary an invention mould, however, 

 terminate here, it is not eafy to believe. Thfi curiofity 

 of the public has for the prefent been fatiated ; and 

 the few accidents which have happened have dimi- 

 nifhed the fpirit of adventure. The difficulty, indeed, 

 of regulating the courfe of thefe aerial machines feems 

 an almoft infurmountable bar to their general utility. 

 But who will prefume to fet bounds to the ingenuity 

 and courage of man ? The firft mortal, who com- 

 mitted himfelf to the waves on a mifhapen raft, had 

 probably no fufpicion of even thofe trivial improve- 

 ments 



