restricted. We then considered cutting in the wall of the cabin a conical 

 hole to which would be fitted a block of glass in the shape of a truncated 

 cone. The visual field would then be much larger. On this subject I 

 consulted Professor Michels in his celebrated laboratory for high 

 pressures at Amsterdam: he advised me against it, predicting cracks 

 when the bathyscaphe rose again to the surface, that is to say, when the 

 pressure was diminishing. I then made other experiments interposing 

 between glass and steel a diversity of softer substances. One day we 

 thought we had won: the glass appeared to have resisted perfectly. 

 But we suffered a great disappointment when two days later there 

 appeared in our block of glass, near the smaller end, a series of scarcely 

 perceptible cracks. It was clear that we could not use glass. 



Then, in May 1939, Professor Guillisen, my young assistant's 

 father, drew my attention to plexiglas, a perfectly transparent organic 

 substance well known today, which had then just appeared on the 

 market. Plexiglas is much less hard than glass: therefore I thought 

 that, to have the slightest chance of success, we should have to reduce 

 the internal diameter of the new porthole to minute proportions. But 

 we discovered during model trials that this was quite unnecessary 

 and that all things considered a porthole with an internal diameter of 

 3-94 in. (10 cm.), an external diameter of 15*75 in. (40 cm.) and 

 5-91 in. (15 cm.) thick was amply strong. By extrapolation from the 

 results of our observations, I felt able to conclude that such a porthole 

 would only be deformed permanently at a pressure corresponding to 1 8 j 

 miles of water, and that failure (which would, of course, be fatal 

 for the occupants of the cabin) would only threaten them at much 

 greater pressures. This result, so surprising at first glance, is explained 

 by the fact that plexiglas is somewhat plastic : if a small part of the 

 substance is overloaded beyond its limit of elasticity, it goes slightly 

 out of shape and passes the excess load to adjacent parts : thus the 

 stresses are distributed in a more uniform fashion throughout the 

 entire piece, while glass, which does not possess this plasticity, can only 

 yield to an overload by cracking. 



These windows are perhaps the finest feature of the bathyscaphe. 

 Let the reader picture to himself an observer stooping to one of 

 these portholes and contemplating with his own eyes the world of 

 the ocean depths which, for the first time, are revealed to man. On 

 one side an interior, of reduced dimensions it is true, but comfortable 

 enough. Here a normal atmospheric pressure prevails. On the other 



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