few hours later. These dangers are lessened either by a very slow rise 

 to the surface, the diver stopping to rest at different levels: or by 

 diving only for very short periods and then rising very rapidly. 

 They would be eliminated completely if it were possible not to 

 breathe at all during the dive. In short, here is a dilemma : if the diver 

 breathes, he is threatened with nitrogen intoxication, but if he is 

 deprived of air, he suffocates. 



From what we have just seen it is clear that the classic methods of 

 diving cannot open the doors of the submarine abysses to man. 

 Human limits are very restricted indeed. Down to 20 fathoms, it is 

 admitted, there is no danger provided the diver ascends slowly, and 

 sports amateurs are recommended not to go beyond this limit. Trained 

 divers often go down to 30 fathoms. But lower than that the danger 

 increases very rapidly. At 45 fathoms serious accidents are to be 

 expected and rare are the cases where a diver has been able to return 

 in good condition from the depth of 50 fathoms. For greater depths, 

 nitrogen has been replaced, in the United States, by helium, which 

 produces no chemical reaction and which is much less soluble in blood. 

 Thus the diver risks neither intoxication nor embolism. This has 

 permitted Bollard to endure a pressure of 100 fathoms of water. But 

 what are these hundred or so fathoms of water in comparison with the 

 miles which measure the ocean depths ! 



A minor difficulty, but nevertheless one which cannot be neglected, 

 is known to many divers. At times they feel a sharp pain in the ears. 

 The same pain can be felt in a free balloon or in an airplane. The 

 explanation is known : the middle ear contains air separated from the 

 external air by the tympanum. There can be too much pressure on 

 this membrane if external pressure increases (or if it diminishes, as is 

 the case when one goes up in a balloon) and the air within the ear fails 

 to adapt itself to this pressure. Fortunately the middle ear is connected 

 with the buccal cavity by the Eustachian tube. If this tube is quite 

 open, the entire dive can take place without pain. But if the tube opens 

 too late, a pain is then produced. By swallowing motions, skilfully 

 executed, in general the canal can be opened. If a courageous diver 

 keeps on with his work without the tube being open, a perforation of 

 the tympanum many result with consequences that can be imagined. 

 The susceptibility of the ear differs not only from man to man, but in 

 the same diver, from day to day. The slightest cold can cause an 

 obstruction of the Eustachian tube. 



