between these two pressures. In the majority of technical structures a 

 safety factor of 4 is insisted upon. If the designer can call upon very 

 great experience in the matter, and above all if the failure of the 

 structure would not be fatal to the crew, a safety factor of 3 or even 2 

 would be permissible. But in our case I thought that it would be better 

 not to go lower than a safety factor of 4. 



Although it immediately appeared probable to us that we should 

 fix our choice of material upon steel, we first of all made some trials 

 with plexiglas and with a magnesium alloy. These materials, lighter 

 than steel, would have allowed us to have a greater wall thickness for 

 the same weight. The magnesium alloy did not satisfy us. As for the 

 plexiglas, it is not strong enough for great depths : on the other hand it 

 would probably do very well for the mesoscaphe, which is not in- 

 tended to go down beyond some 500 fathoms (see page 142). 



We therefore chose a steel of the best quality, in this instance the 

 cast-steel called 'indefatigable' from the Henricot Steel Mills (Aciéries 

 Henricot) of Court-Saint-Etienne, Belgium. In consequence of our 

 calculations and trials with models, we gave the walls a thickness of 

 3-54 in. (9 cm.), increased to 5-91 in. (15 cm.) in the neighbourhood 

 of the openings. Thus conceived, the cabin would probably be crushed 

 at a pressure of 10 miles of water: at 2^ miles deep we should then 

 have what we want, a safety factor of 4. 



However, it can happen, above all when pieces of cast metal are 

 used, that bubbles and flaws form and remain in the interior of the 

 metal without there having been any fault on the part of the foundry- 

 man. When mills proceed to mass production they' make preliminary 

 trials in order to determine the casting conditions which will give them 

 an absolute guarantee of the homogeneity of the metal. In our case 

 this was not possible. Therefore we subjected the finished cabin to 

 radiographic examination. ^ The Union Minière du Haut- Katanga 

 (Union of Mines of Upper Katanga) lent us a gram of radium, which 

 we placed in the centre of the sphere. Photographic films, in the 

 aggregate 1 8 square yards, were laid around the external surface of the 

 cabin. After being exposed for twenty-four hours, the developed film 

 showed that the steel was homogeneous almost throughout. In 

 certain spots, however, we discerned lacunae in the material. To dis- 

 cover their nature, we then proceeded, with the help of a sort of boring- 

 chisel, to take a sample of the defective part : the material thus removed 



1 With the assistance of the Etablissements Gevaert. 



[42] 



