MIRACLES OF SCIENCE 



driven by propellers not unlike those of a steamship 

 (those of the Parseval, however, having- canvas 

 vanes), and depend for their dirigibility upon hori- 

 zontal and vertical rudders, the former of which are 

 not unlike the wings of an aeroplane. 



The Parseval airship is supplied with an ingenious 

 internal mechanism, which serves the double purpose 

 of aiding in steering the ship and of compensating the 

 loss of gas, which is always a serious item in a long 

 voyage. This apparatus consists of two airbags or 

 ballonets, within the main body of the balloon, 

 capable of being independently filled with air or ex- 

 hausted. If the forward ballonet is filled with air 

 and the rear one deflated, the prow of the balloon is 

 thereby made heavier and tends to head downward; 

 and of course the conditions are reversed if the front 

 ballonet is deflated and the rear one filled with air. If 

 so much hydrogen is lost from the balloon that there 

 is danger of collapse, both ballonets may be inflated 

 and the loss thus compensated. 



When it is recalled that air is about fifteen times 

 heavier than hydrogen, it will be obvious that the air 

 pumped into the ballonets serves as ballast. The 

 use of air for this purpose seems to mark an im- 

 portant step in the art of aeronautics. The recog- 

 nized form of ballast in the early day of ballooning 

 was the sand bag. This serves a useful purpose in 

 lightening the balloon, but the sand once thrown out 

 obviously cannot be replaced. What is needed is a 

 form of ballast that can be taken on or thrown out at 

 will. In the day time the sun heats the gas bag and 

 the balloon expands and tends to rise. At night the 



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