We know, in fact, that any piece of metal can always, because of lack 

 of homogeneity or because of some internal tension, yield to stresses 

 less than the calculated ultimate strength. It is the province of the 

 engineer, and his alone, to work out the load which may be sustained 

 without abnormal risks. For my own part, I considered that depths 

 up to 2^ miles could be reached practically without any danger by 

 our cabin, seeing it had been made of finest quality forged steel. I 

 should have been much less categorical if we had been dealing with a 

 cabin in cast steel. 



To go down deeper it would be necessary first to carry out trials 

 with the bathyscaphe empty, that is to say, to install an automatic 

 pilot in the Trieste as we had done for the FNRS 2. We already had 

 the necessary instruments. At the moment when the bathyscaphe 

 reached the depth intended, two pressure gauges, independently of 

 each other, would cut the current feeding the magnetic valves and 

 would thus automatically start the ascent. If for any reason the bathy- 

 scaphe rested on the bottom before reaching the depth intended, two 

 pieces of mechanism, independently of each other, would cut the 

 current to the magnetic valves, after a certain set time. Finally, if a leak 

 occurred and salt water entered the cabin, it would close an electric 

 circuit and by means of a servo-mechanism would cut the current 

 to the magnetic valves. 



These empty trials would greatly increase the safety. Modern 

 industry, which makes great use of this method, generally carries them 

 out with overloads of 50%. I know only one case where the empty 

 trial is impracticable: it is that of dams for hydro-electric systems. 

 Even if it were possible it would call for the evacuation of hundreds of 

 thousands and in certain cases of millions of inhabitants. Military sub- 

 marines indeed are not generally subjected to empty trials. It would, 

 however, be easy to fix under the hull ballast to be dropped auto- 

 matically for the trial dive. 



What is the depth that the Trieste could reach after having been 

 successfully through empty dives ? It is impossible to say precisely : 

 it depends on the aim of the dive and above all on the risk that 

 the crew has agreed to take. As for me, I was willing to go down 

 as far as 3f miles without any fear, not to beat a record, but to make 

 observations, the scientific value of which would be recognized 

 beforehand. 



If it were desired systematically to explore the very great deeps it 



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