186 "WELLS'S NATUEAL PHILOSOPHY. 



As the density of the air continuaUj diminishes as we 

 wiUan^sSnd- ^cend from the surface of the eartli, it is evident that such 

 ing body re- a body, as it goes up. will finaUv attain a height where the air 

 ^jy , °' will have the same density as itsel£ and at such a point the 



body will remain stationary. Upon this principle clouds, at 

 different times, float at different degrees of elevation. 



It is also upon these principles that aerostation, or the art of navigating the 

 *ir, depends. 



What are Bai- 398. BallooHS Ere macliines which ascend 

 loons? through the atmosjjhere, and float at a certaia 

 height, in virtue of being filled with a gas or air lighter 

 than the same bulk of atmospheric air. 

 -^ ^ ^^ Balloons are of two kinds. Montgolfier, 



What are the ' 



two varieties or rarefied air balloons, and Hydrogen gas 



of balloons? in rn 



balloons. The first are filled with common 

 air rarefied by heat, and thus made lighter than the 

 surrounding atmosphere ; while the second are filled 

 with hydrogen, a gas about fourteen times lighter than 

 air. 



Describe the '^^ rarefied air-balloon was invented by Montgolfier, a 



Montgolfier. or French gentleman, in 1782, who first tilled a paper bag with 

 l^ll^^^ "^'^ ^^^' tieated air, and allowed it to pass up a chimney. He after- 

 ward constructed balloons of silk, of a spherical shape, with 

 an aperture formed in the lower surface. Beneath this opening a hght vriie 

 basket was suspended, containing burning material The hot air arising from 

 the burning substances, enters the aperture, and rendering the balloon specific- 

 ally Hghter than the air, causes it to ascend with considerable velocity. 

 Small balloons of a similar character are frequently made at the present day 

 of paper, the air within them being rarefied by means of a sponge soaked in 

 alcohol, suspended by a wire beneath the mouth, and icmited. 

 Describe the "^^^ hydrogen gas balloon consists of a light silken bag, 



hj-drogen gas filled either with hydrogen, or common illuminating gas. The 

 balloon. difference between the specific weight of either of these gases 



and common air is so great, that a large balloon filled with them possesses 

 ascensional power sufficient to rise to great heights, carrying with it consid- 

 erable additional weight. The aeronaut can descend by allowing the gas to 

 escape by means of a valve, thereby diminishing the bulk of the balloon. To 

 enable him to rise again, ballast is provided, generally consisting of bags of 

 Band, by thro^ving out which, the balloon is Hghtened, and accordingly 

 rises. 



By means of one of these machines Gay Lussac, au eminent French chem- 

 ist, ascended in 1804. for the purpose of making meteorological observations, 

 to the great height of 23,000 feet. 



