Appendix y 



Different Methods of Checking the Homogeneity 

 of the Metal in the Cabin 



Modern industry employs two methods of checking the internal 

 structure of a casting : X-rays and gamma rays on the one hand and 

 ultrasonic techniques on the other. 



The first method produces an image of the object on the fluorescent 

 screen or on the photographic film. Small airholes of 2V iri- ii^ dia- 

 meter or more, by the fact that they are more permeable than the metal, 

 appear as light marks on the screen and as dark spots on the photo- 

 graphic plate. This method particularly lends itself to the study of 

 castings. But the cabin of the Trieste was forged. A possible gas bubble 

 or speck of slag by this operation would have been flattened into a 

 flake or fault parallel to the surface of the sphere. The thickness of this 

 would have been very slight. The difference of absorption of rays be- 

 tween the sound parts of the hull and the defective part would not have 

 been perceptible. Nevertheless the management at Terni particularly 

 wanted to make a complete radiograph of the cabin. A foreign body 

 which had not been flattened by the forge would have shown up on it. 

 The photographic films obtained were uniformly grey, without the 

 slightest clouding. From this point of view, we were completely 

 reassured. 



The second method, that of the ultrasonic technique, gave proof 

 that there were no faults parallel to the surface. 



This method has been in use in industry only for a few years. The 

 principle is very simple. Let me explain it in a little digression. We are 

 in a free balloon, the night is very dark, visibility nil. The pilot would 

 like to know at what distance he is above the ground. Is he in danger 

 of crashing into a mountain.'^ Nothing is simpler than to make an 

 approximate sounding of the distance from the ground. A strident 

 cry or a trumpet blast: after a few seconds, the terrestrial echo will 

 send him back the sound. Each second roughly is equal to 500 ft. 

 (since the speed of sound in the air is of the order of 1 100 ft. per second). 

 If there is a discontinuity of temperature in the air, an intermediate 

 echo is very distinctly perceived. This same method, it is true with 

 certain modifications, is used to check materials. There is then 

 employed a source of sound of very high frequency (a piezo-electric 



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