84 CONSTRUCTION OP BALLOONS. 



then a leaden tube, to the neck of the vessel ; and let the end 

 of the tube be so shaped that going into the water it may 

 terminate under a sort of inverted vessel, to the upper aper- 

 ture of which the balloon is adapted. Things thus prepared, 

 if the part of the vessel is put into the fire, and made red. hot, 

 the inflammable air produced will come out of the tube, and 

 passing through the water will at last enter into the balloon. 

 Previous to the operation, as a considerable quantity of common 

 air remains in the inverted vessel, which it is more proper to 

 expel, the vessel should have a stop-cock, through which the 

 common air may be sucked out, and the water ascend as high as the 

 stop.cock. To procure inflammable air by means of steam, Dr. 

 Priestley used a tube of red.hot brass, upon which the steam of 

 water has no eft'ect, and which he rills with the turnings of iron that 

 are separated in the boring of cannon. By this means he obtained an 

 inflammable air, the specific gravity of which is to that of common 

 air as 1 to 13. In this method, not yet indeed reduced to general 

 practice, a tube about throe-quarters of an inch in diameter, and 

 about three feet long, is filled with iron turnings ; then the neck of 

 a retort, or close boiler, is luted to one of its ends, and the worm of 

 a refrigeratory is adapted to its other extremity. The middle part 

 of the tube is then surrounded with burning coals, so as to keep 

 about one foot in length of it red. hot, and a fire is always made 

 under the retort or boiler sufficient to make the water boil with ve- 

 hemence. In this process a considerable quantity of inflammable 

 air comes out of the refrigeratory. It is said that iron yields one 

 half more air by this means than by the action of vitriolic acid. 

 With regard to the rarefied.air balloons, the method of filling them 

 is by means of a scaffold, the breadth of which is at least two-thirds 

 of the diameter of the machine, and elevated about six or eight feet 

 from the ground. From the middle of it descends a well, rising 

 about two or three feet above it, and reaching to the eround, fur. 

 nished with a door, through which the fire in the well is supplied 

 with fuel. The well should be constructed of brick, and its dia- 

 meter somewhat less than that of the machine. On each side of the 

 scaffold are erected two masts, each of which is fixed by ropes, and 

 has a pulley at the top. The machine is to be placed on the sraf- 

 fold, with its nock round the aperture of the well. The rope pas. 

 sing over tlu- pullvys of tlte two masts, serves to lift the balloon 

 about fifteen feet above the scaffold ; and it is kept steady, and held 



