The Polar Airship 



209 



In the central zone embracing the 

 "mai-tre couple," or greatest diameter, the 

 pressure of the gas rises to 466 kilos per 

 square meter, equal to about 95 pounds 

 per square foot. It is upon this central 

 zone the envelope is applied as out- 

 lined above — one strong silk and two 

 thicknesses of cotton, with three coats 

 of rubber. These three thicknesses of 

 material, consolidated into one, give a 

 total tensile strength of 2,800 kilos per 

 square meter, or about 575 pounds per 

 square foot. Hence we have this re- 

 sult: Maximum strain, 95 pounds per 

 square foot; tensile strength, 575 

 pounds per square foot; coefficient of 

 safety, 6 to 1. 



In the next zones the pressure ranges 

 from 315 to 450 kilos per square meter. 

 With a maximum of 450 kilos to pro- 



--C 



vide for, a lighter silk is used in these 

 zones, reducing the weight of the en- 

 velope to 455 grammes per square me- 

 ter, but retaining 2,400 kilos of tensile 

 strength, which means a coefficient of 

 more than 5 to I. 



In the outer sections the maximum 

 pressure is 350 kilos, and here the en- 

 velope is composed of two thicknesses of 

 cotton with three coatings of rubber, 

 omitting the silk, and again saving in 

 weight, but securing 1,800 kilos of 

 strength per square meter — again with 

 a coefficient of safety of more than 

 5 to 1. 



the tensile; strength 



In the Lebaudy airship the coefficient 

 of safety was 3V2 to 1. We have a co- 

 efficient of more than 5 to 1 throughout. 



The tensile strength of the fabrics is 

 not a matter of guesswork. Samples of 

 each consignment from the manufac- 

 turer are submitted to the Paris Cham- 

 ber of Commerce, tested by dynamom- 

 eter, and officially stamped. These 

 tests are under the regulations of the 

 chamber of commerce, and the certifi- 

 cates are made the bases of contracts 

 and their fulfillment. 



In computing the work of the gas 

 upon the fabric of the balloon M. God- 

 ard and his engineers have assumed 

 that the interior pressure is equivalent 

 to 30 millimeters of water, or about 6 

 pounds per square foot. This pressure 

 is maintained by means of the venti- 

 lator or blower which inflates with air 

 the balloonet or interior balloon, and 

 which is operated by an independent 

 motor of 5 horsepower in the engine- 

 room. The use of the pressure is to 

 maintain the rigidity of form of the 

 great balloon, as there are no interior 

 frames or other stiffening devices. The 

 integrity of form is maintained solely 

 by interior pressure, and this pressure 

 is usually at 20 to 24 millimeters. In 

 taking 30 millimeters and adding it to 

 the upward thrust or lifting force of the 

 gas itself (somewhat more than one 

 ounce per cubic foot), M. Godard has 

 shown the conservatism which charac- 

 terizes all his calculations and work. 

 Though it is unlikely the interior press- 

 ure within our balloon will ever exceed 

 25 millimeters, the fabric of the envel- 

 ope has strength sufficient to give a 

 factor of safety of 5 to I at 30 milli- 

 meters. 



In addition to the tensile strength of 

 the envelope, every seam, whether cir- 

 cumferential or longitudinal, is rein- 

 forced. The material is lapped about 

 25 millimeters (one inch), and doubly 

 sewn. Inasmuch as there is danger 

 that the hydrogen may escape through 



