The use of aviation gasoline does pose some problems 

 in that it is more compressible than water. Since its steel 

 shell is rigid, the float has to be pressure- compensated to 

 avoid its being crushed as the gasoline is compressed during 

 descent. As the craft descends into the sea, a two-way 

 "breathing" valve fitted in the float opens inward and allows 

 sea water to enter the float. The sea water, being more 

 dense than the gasoline, sinks to the bottom of the float. 

 As the craft returns to the surface and the outside pressure 

 decreases, the valve opens outward and the expanding gaso- 

 line forces the sea water back out through the valve. In 

 this way, the thin-shelled balloon is pressure-compensated 

 at all times. The compressional loss of buoyancy is of 

 such a magnitude that approximately 1 ton of ballast must 

 be dropped for every 3000 feet of descent to maintain equilib- 

 rium. 



The cabin (sphere) suspended beneath the balloon of the 

 bathyscaph is quite similar to the cabin of an airborne bal- 

 loon. The prime difference is that the latter is designed to 

 maintain atmospheric pressure inside as the external pres- 

 sure decreases, while the former is designed to maintain 

 atmospheric pressure as the external pressure increases 

 (up to 16, 000 psi at 35, 800 feet). The sphere has thick 

 enough walls to maintain atmospheric pressure at all times 

 irrespective of depth. The sphere is the only pressure- 

 resistant assembly on TRIESTE; all the other devices are 

 pressure-compensated. The reason for pressure-compen- 

 sating the other devices is as follows. In TRIESTE, all 

 buoyant force is derived from the lift generated by the 

 fixed quantity of gasoline; this lift decreases (through 

 compression) with the depth. Subtracting the constant 

 (structural) weight of the craft from the variable lift, we 

 are left with a variable payload. Therefore, any reduction 

 of the structural weight will be repaid in ability to carry 

 more payload. 



For diving, it is necessary to make the craft only 

 slightly negatively buoyant, which is done by flooding the 

 two small ballast tanks, one located at each end of the 

 float. These tanks operate in a similar fashion to ballast 

 tanks on submarines. 



Prior to going to sea, the bathyscaph is given an 

 equilibrium test to determine its exact buoyancy trim. An 

 actual test dive is made alongside a pier to determine how 

 much negative buoyancy the craft will have when the end 

 tanks are flooded. The trimming of the craft is accom- 

 plished through external addition or subtraction of lead 



