From the moment the bathyscaphe was begun it was necessary to 

 know what quantity of ballast it must carry. An excess of 

 ballast is useless and increases the cost of building and use of the 

 submarine, since it necessitates an increase in the volume of the float. 

 But if the quantity of ballast available is insufficient, woe to the pilot! 

 Without going into details, we shall trace the principal elements of the 

 problem. 



If a float of roughly 22,000-gallons capacity, as that of the Trieste 

 for example, is in equilibrium on the surface with the sea water at 

 86° F. and has this temperature itself, its lifting capacity diminishes, 

 in round figures, by 8800 lb. (4000 kg.) if it goes down to 2^ miles 

 (4 km.) and reaches layers where the temperature is around 32° F. and 

 if it stays long enough at this depth to reach this temperature itself. 

 But if the descent is very rapid, the petrol, instead of getting cooler, 

 is heated by the effect of the compression : this heating is in the order 

 of 4*5° F. per 3300 feet, in difference of level. 



This is the phenomenon called 'adiabatic heating'. We shall benefit 

 by this heating when the bathyscaphe, steered by an automatic pilot, 

 will descend rapidly to 3I; miles. To compensate the decrease of 

 buoyancy we must then dispose of a quantity of ballast of 8800 lb. 

 To this ballast, which is obligatory, it is advisable to add what we shall 

 call emergency ballast, which we fixed at an amount of 9460 lb., 

 because if, during a test when empty, a joint breaks and the cabin is 

 entirely flooded with water, it will suffer an overload cf 9460 lb. : and 

 we want the bathyscaphe to rise again to the surface in spite of this 

 overload. 



The float of the Trieste^ as a safety measure, was divided into com- 

 partments, the largest of which had a volume such that the loss of 

 buoyancy would not exceed 9460 lb. if, following upon serious 

 damage, all the petrol were replaced by water. 



Other considerations likewise prescribe emergency ballast: in 

 particular, the danger of being stuck in the bottom mud or in other 

 obstacles such as seaweed. Likewise, in case of danger, we had to be 

 able to unballast the bathyscaphe completely with the minimum delay 

 to be able to go up at full speed. It is clear that these last considerations 

 cannot be calculated in advance. But I estimated that 9460 lb. of emer- 

 gency ballast would be sufficient. 



The reader who is concerned for our safety will ask: 'What would 

 happen if several of the mishaps mentioned occurred at the same time.'* 



[34] 



