side the ocean, which is weighing on the external surface of the 

 plexiglas with a force of some 500 tons. Our confidence in these 

 windows was so complete that during all our dives we never thought 

 for an instant of the consequences of our plexiglas cracking. The 

 production of such items is not in the normal programme of industrial 

 manufacture. Therefore we owe our suppliers ^ special gratitude for 

 the care they took in furnishing us with these beautiful objects, whose 

 transparency is that of pure crystal. 



It should be added that observation can be improved and the visual 

 field of one of these portholes can be greatly increased by applying 

 a prism of plexiglas to the internal surface : the total reflection that the 

 thin layer of air separating the two parts would produce may be 

 prevented by introducing a drop of glycerine.^ 



THE JOINTS 



It is not enough that the cabin should be solid : il is also necessary 

 that the different joints and particularly the great joint between the 

 two hemispheres should be perfectly tight. 



There are many methods in use to make sure that the joint between 

 two rigid parts does not leak. When there is low pressure, of the 

 order of one atmosphere, for example, it is enough to place a layer 

 of rubber between the two parts. Because of its flexibility, the rubber 

 adheres perfectly to the two parts, even if these are not perfectly flat : 

 this is the sealing commonly used in preserving jars. At higher pressures 

 the rubber ring is in danger of being forced out of its position. The 

 rubber must then be replaced by a harder substance, leather, lead, or 

 even pure copper, and the parts of the vessel must be clamped strongly 

 together by means of powerful bolts. However, even for very high 

 pressures one can, in certain cases, use a rubber ring embedded in a 

 groove. 



In the case of our two hemispheres the interposition of any more or 

 less flexible or plastic washer would offer great dangers. One cannot 

 in fact prevent two hemispheres forced against each other by the 



1 When the Trieste was building, it was the Société Vétrocoke at Porto- 

 Marghera (Venice) which was kind enough to supply us with three new port- 

 holes in plexiglas. 



2 The reader wanting more precise details will find in the appendices a résumé 

 of the pressure tests that we made in 1938 with plexiglas models in different 

 shapes. 



[46] 



