THE BALLOON. ** 223 



, which has half the weight of air, its contents will 



1131 

 weigh ^ = 565 kgr> 5, and hence the balloon will rise 



if the envelope with all appendages weighs less than 

 565 kgr '5. If the envelope weighs 300 kgr , the weight of 

 the balloon is 565*5 + 300 = 865 kgr '5, which is 

 1,131 - 865-5 -265 kgr -5 less than the weight of an 

 equal volume of air ; the balloon will therefore rise 

 with a force of 26 5 kgr * 5, and may yet be weighted by 

 a network, a light wicker- work boat, and two men. 

 Large balloons are usually made of bands of silk, sewed 

 together, and covered with caoutchouc varnish, which 

 renders the whole air-tight. For small balloons very 

 thin light membranes must be used, if they are to rise. 

 A balloon of 1 litre capacity must have a weight of not 

 quite O gr '625 if it is to rise, when filled with coal-gas; 

 for 1 litre of water weighs 1 ,000 gr , 1 litre of air weighs 

 therefore 1-^ x 1,000 x l gr -25, and an equal volume of 

 coal-gas weighs 0^-625 ; hence, if the balloon when filled 

 is to be lighter than the displaced air, the envelope 

 must weigh less than T25 0'625 = 0'625 grammes. 

 A balloon of the same size, but filled with hydrogen, 

 would, however, rise very well even if the weight of 

 the envelope exceeded 1**; for hydrogen weighs less 

 than yL of air; 1 litre of it therefore weighs not 

 quite 1-25 x T L= O gr -089; the filled balloon would 

 thus weigh IF-089, hence 1*25 - R089 = 0^161 less 

 than the air displaced by it. 



Where coal-gas is accessible, soap-bubbles filled with it may be 

 easily obtained. Pass one end of an india-rubber tube over a burner, 

 (as in fig. 20, page 17), and insert into the other end the tube of 

 a clay tobacco-pipe, or of a small glass funnel which is not more 

 than 3 cm wide at the mouth. Dip the mouth of the funnel or of the 





