390 Introduction to the Study of Science 



I. BALLOONS 



178. Early balloons. The first type of balloon with which 

 experiments were made was the hot-air balloon. The familiar 

 toy balloon is usually of this type. The air inside the balloon 

 is expanded by heat and is therefore less dense than the outside 

 air. Hence the balloon is forced upward and will remain in the 

 air as long as the temperature difference is maintained. Such a 

 balloon was made to carry one or two passengers, but failed to 

 render any valuable service. 



In the latter part of the eighteenth century hydrogen was 

 discovered (page 117) and the means of producing it in quantities 

 was devised. This made ballooning more practicable. Then 

 the discovery was made that illuminating gas, which contains a 

 large percentage of hydrogen and carbon monoxid, could be 

 substituted for hydrogen ; but because its average density is 

 greater than that of hydrogen, it had only a limited use. 



Balloons were not under control when in the air, save in 

 ascending and descending. They drifted with the wind. By 

 throwing out ballast the balloon could be made to ascend ; and 

 by opening a valve to let hydrogen escape, it could be made 

 to descend. Balloon ascensions and drifting races furnished 

 some excitement for the spectators and adventure for the 

 aeronaut. Such balloons are not of great utility, however, 

 except in the investigation of the atmosphere, its move- 

 ments, density, temperature, and amount, at different high 

 altitudes. 



179. Use of balloons in study of atmosphere. A few 

 important facts about the atmosphere which were discovered 

 by means of the balloon may be mentioned in this connection. 

 Two English aeronauts in a gas balloon attained the record 

 height of seven miles. They found the temperature at this 

 altitude about 60 F., and the pressure only one fourth that 

 at sea level. They nearly perished in their adventure. With 

 their limbs paralyzed by cold and weakness, they owed their 



