CONSTRUCTION OF BALLOONS. QJ 



riown, whilst fillin, by ropes passing through loops or holes about 

 its equator ; and these ropes may easily be disengaged from the 

 machine, by slipping (hem through the loops when it is able to MIS. 

 tain itself. The proper combustibles, to be lighted in the well, are 

 those which burn quick and clear, rather than such as produce mm h 

 smoke; because it is hot air, and not smoke, that is required. Small 

 wood and straw are very fit for this purpose. As the current of hot 

 air abends, the machine will dilate, and lift itself above the scaffold 

 and gallery which was covered by it. The passengers, fuel, instru- 

 ments, &c. are then placed in the gallery. When the machine makes 

 efforts to ascend, its aperture must be brought, by means of the ropes 

 annexed to it, towards the side of the well a little above the scaffold ; 

 the fire-place is then su*pencid in* it, the fire lighted in the grate, 

 and the lateral ropes being slipped off, the machine is let go. It has 

 been determined by accurate experiments, that only one. third of the 

 common air can be expelled from these large machines; and there- 

 fore the ascending power of the rarefied air in them can be estimated 

 as only equal to half an ounce avoirdupoise for every cubic foot. 



The conduct of balloons, when constructed, filled, and actually 

 ascended in the atmosphere, is an object of great importance in the 

 practice of aerostation. The method generally used for elevating 

 or lowering the balloons with rarefied air, has been the increase or 

 diminution of the fire ; and this is entirely at the command of the 

 aeronaut, as long as he has any fuel in the gallery. The inflamma- 

 ble.air balloons have been generally raised or lowered by diminishing 

 their ballast, or by letting out some of the gas through the valve : 

 but the alternate escape of the air in descending, and discharge of 

 the ballast for ascending, will by degrees render the machine inca- 

 pable of floating ; for in the air it is impossible to supply the loss of 

 ballast, and very difficult to supply that of inflammable air. These 

 balloons will also rise or fall by means of the rarefaction or con. 

 densalion of the inclosed air, occasioned by heat and cold, as has 

 been already observed. Wings or oars are the only means of this 

 sort that have been used with any probable success ; and as Mr, 

 Cavallo observes, they seem to be capable of considerable improve, 

 ment, though much is not to be expected from them, when the 

 machine goes at a great rate, it is a matter of surprise, that the 

 various hints for directing balloons appear to lie dormant with 

 their projectors who seem indisposed to make any attempts to 

 carry their plant into execution : thus the inventions of professor 



a 



