AIRSHIPS AND AIRPLANES 303 



the holes in the stopper. Put a thistle tube through the other 

 hole. Put the stopper in the bottle. Push down the thistle 

 tube nearly to the bottom of the bottle. Add water till it 

 rises to cover the end of the thistle tube. Push one end of a 

 rubber tubing over the end of the right angle tube. Make some 

 soapsuds in warm water. Dip the end of the rubber tubing 

 into the soapsuds. Add hydrochloric acid through the thistle 

 tube. Hydrogen will be generated and will pass out through 

 the tube into the soapsuds. Remove the tube from the suds 

 and allow a bubble to form on the end of the tube. Shake it 

 off. Why does it rise? This same gas is often used to fill 

 balloons. Apply a lighted match to a floating bubble. 



Balloons for military purposes. Balloons were used for 

 military purposes during the Civil War, serving as a van- 

 tage point from which the enemy's works could be seen. 

 Since that time all the large countries have made some use 

 of balloons as a part of their military equipment. 



Dirigible balloon. The next great step in the develop- 

 ment of air navigation was the appearance of the dirigible 



/Gas bag 



Po/nfs of \ 

 support 



Petro/ to/ik' / I \ \Er>(j/ne ~Dr y'v er^S/eersinon Car Rudder- 

 Exhaust' \ Choin drive 

 Propetfer 



FIG. 121. Longitudinal view of typical dirigible balloon. 



balloon, or airship. This was propelled by artificial means 

 and steered by a rudder. The first attempts were not very 

 successful on account of the great weight of the engines 

 used, and it was not until light engines of great horse power 



