CHAP, v.] BALLOONS. 91 



II. MONTGOLFIKRES, OR HuT-AIR BALLOONS, AND GAS-BALLOONS 

 CONSTRUCTION AND FILLING. 



Balloons, whether filled with hot air or gas, are generally of a 

 nearly spherical form terminated at the lower part by a cylindrical or 

 conical appendage, There is always this difference, that in the air- 

 balloon this appendage, called the neck of the balloon, has an opening, 

 whilst in the gas-balloon it is closed. This form is moreover that 

 which the envelope would naturally take under the pressure of the 

 elastic gas which it incloses, if it were equally extensible in all its 

 parts. When in the air the orifice in the neck of the gas-balloon is 

 always open, as in Fig. 56 ; it is' only closed during inflation, to 

 prevent the escape of the gas. The only difference between the air 

 and gas-balloon is, that in the former the orifice is very large, as the 

 stove chimney has to go up through it and be well separated fiom the 

 material of the balloon, and in the latter the orifice does not exceed 

 a foot in diameter. 



The envelope is formed of spindle-shaped pieces of silk, which 

 are sewn together, as it were along the meridians of a sphere ; it is 

 important that no fissure is left, not even the holes made by the 

 pricks of the needle, and that the stuff itself should be of a close 

 texture and if possible impermeable, to avoid escape of gas, which 

 would diminish the ascending power. Montgolfier used in his first 

 experiment a cloth lined with paper, sewn on a network of string, 

 and fastened to it ; in his second experiment, the envelope was of 

 packing cloth, lined inside and outside with very strong paper. We 

 have seen that Charles's balloon was of silk and covered with a 

 varnish of indiarubber. The balloon that MM. Barral and Bixio 

 used for their two explorations in 1850, was rendered impermeable by 

 a coating of linseed oil thickened with litharge. Lastly, another good 

 way of construction consists in placing a sheet of indiarubber 

 between two sheets of silk. 



The upper part of a balloon is covered with a net which hangs 

 loose a little below its equator ; all the cords of this net are brought 

 down to a circle of very hard wood which serves to suspend the car. 



Thanks to this arrangement, the weight is evenly spread on the 



