THE CONQUEST OF THE AIR. 349 



balloon should be able to travel at a speed of about 50 miles per 

 hour. It will then be available under practically all conditions of 

 the M^eather. In other words, it should have as much power as 

 would drive a boat, carrying- the same weight, 25 miles an hour 

 or should have the same ratio of power to size as the Lusitania. 

 In the construction of dirigibles the outstanding points are, there- 

 fore, small air resistance, by suitably shaping the balloon, large 

 motive power at a small weight, excellent steering facilities, and 

 stability. 



As a matter of fact, dirigible balloons became a possibility 

 with the advent of the benzine motor. It is now possible to obtain 

 motors which weigh only i|^ to 2 kgs. per horse power. Even 

 in the year 1900, the weight was still 20 to 25 kgs. per H.P., so 

 that an enormous success in this direction has to be recorded. 



The present-day dirigibles are divided into three classes : The 

 non-rigid, semi-rigid, and rigid types. Perhaps the best way of 

 studying them is by describing one of each class. 



One of the most successful non-rigid airships is the one due 

 to Major Parseval. It has the correct shape : An egg-like prow 

 and a pointed tail. The envelope consists of gores goo mms. 

 wide, made in a similar manner as for the free balloons. The 

 colour of the envelope is again yellow to prevent the passage of 

 the oxidising rays, and the consequent deterioration of the rubber 

 on the inside of the covering. The two layers of cotton cloth 

 are laid diagonally to one another, so that the warp of the one 

 may resist ripping in the woof of the other and localise injuries 

 to the fabric. Just below the Equator of the ship is fixed a belt 

 which carries the gondola, or car, and the propulsion mechanism. 

 In order to keep the balloon always taut, two air bags or ballonets 

 are placed inside the envelope. Air is constantly pumped in,to 

 them, escaping through a valve if the pressure exceeds 20 kgs. 

 to the square metre. As soon as the pressure on the inside of 

 the covering exceeds 10 kgs. to the square metre, the balloon is 

 able to carry the cag-e and its equipment. The covering is of 

 such strength that a bursting will not take place until the pressure 

 exceeds 450 kgs. to the square metre. Hence sufficient margin 

 is left for safety. If the gas is expanded by a rise in temperature, 

 the ballonets are squeezed until the pressure is normal again. On 

 the other hand, if the gas contracts or leaks, the ballonets swell 

 out until equilibrium is restored. 



It will be obvious from this that the proper working and size 

 of the ballonets is of vital importance to a non-rigid balloon. The 

 amazing number of accidents which befel Santos Dumont were 

 nearly all due to insufficient size of the ballonet and the small 

 quantity of air that he could pump into it. The gas contracted 

 more rapidly than the air could be pumped into the ballonets. A 

 collapse was the inevitable result. 



In addition to the air bags, the ship must be provided with 

 safety valves. When the ballonets are almost squeezed flat, and 

 the gas keeps on expanding, a safety valve, manipulated by 

 means of ropes by the bags themselves, automaticallv opens and 

 allows gas to escape. The valve may also be opened by hand. 

 The two ballonets in the Parseval airship serve also for mak- 



