252 SYMPOSIUM ON AERONAUTICS. 



ture variations cause expansion and contraction of gas and con- 

 sequent changes in buoyancy. We may expect the air to grow at 

 least 0.5° colder for each 100 meters rise, but this is rather an average 

 than a normal condition. 



12. Only at night is the gas at the same temperature as the air, 

 for the sun's heat on the balloon keeps the gas inside 10 to 20 degrees 

 warmer. A cloud which cuts ofif this radiation will cause a con- 

 traction of the gas enough to cause a descent. It is of great im- 

 portance to check temperature changes in the gas. Airships have 

 been given metallic flake paints, and light colors in an effort to re- 

 duce heating. The most effective means would appear to be a double 

 wall with air space as in the Zeppelin type. Aluminum paint was 

 found to reflect fourteen times as much radiant heat as unprotected 

 rubber. 



13. Rubberized fabric has been the envelope material for nearly 

 all dirigibles except Zeppelins. Such fabric can be obtained in 

 quantity and of uniform quality. Unfortunately the chemical action 

 of light causes the rubber to deteriorate. Protective coatings of 

 chrome yellow have been used with fair success. More recently 

 carbon black has been found to protect the rubber better. But a 

 dark envelope exaggerates the disturbances of equilibriuiu due to 

 heating. 



14. The hydrogen leakage through good rubberized fabric should 

 be about 9 liters per square meter per day. Goldbeater's skin, which 

 is animal intestine, tanned, shows a leakage of but a quarter of a 

 liter. Such a membrane is immensely superior as a hydrogen con- 

 tainer and does not oxidize. However, gold-beater's skin rots if 

 wet, is difficult to work and to obtain in quantity. It is to be hoped 

 that a hydrogen-tight material can be developed equal to gold- 

 beater's skin but without these disadvantages. 



15. The envelope of a dirigible is a nonconductor of electricity, 

 but presumably picks up the electro-static potential of the air. Ex- 

 periments with kites have shown a potential difference of 20,000 

 volts at 1,000 meters. It is likely that an airship takes up the 

 potential of the air in less than a second and cannot reach the ground 

 even after a rapid descent with any very considerable charge. The 

 potential gradient may be 50 volts per meter and a dirigible of 20 



