controls the heading and the progress of the bathyscaphe. The pro- 

 pulsive groups of the Trieste were placed on the ship's deck, and con- 

 sequently were easy of access when the bathyscaphe surfaced : the 

 fact that here they become unusable is without importance, since one 

 does not have to use them then. In the French bathyscaphe FNRS 3 

 the propellers were placed lower down, beside the float: this is more 

 in conformity with tradition : but in the position so chosen they were 

 more exposed to damage by contact with boats or other obstacles. 



What conditions were necessary for our propelling apparatus ? To 

 make observations, we did not need great speed ; I thought a speed of 

 4 in. per second would amply suffice; or, in nautical language, about 

 J knot. But it must not be lost sight of that the propelling force 

 developed by the screws had to be sufficient to overcome the friction 

 produced by the trail-rope dragging along the ground. As the power 

 of the motors was limited by the capacity of our accumulators, we 

 had to have propellers of large diameter and of low rotational speed. 



How were the motors to function under water? Three methods 

 presented themselves. 



One could consider using three-phase induction motors which have 

 no brush. This was rejected ; we should have had to install in the cabin 

 a rotary converter, which would have been too cumbersome. 



A motor could have been installed, in the air, within an airtight 

 container resisting pressure. We rejected this idea as much on account 

 of the weight and cost of the airtight container as on account of the 

 construction problems that stuffing-boxes present. 



The third solution, the one we chose for the Trieste, is ideal from 

 every viewpoint. The motor turns in a vessel filled with trioline (an 

 organic insulating liquid heavier than water) : the shaft of the motor is 

 vertical and it projects above the vessel. Because of the great density 

 of the insulating liquid, we dispensed with a stufling-box. At the 

 top of the shaft a system of gears produced the desired reduction of 

 speed for the rotation of the screw, and converted it without difficulty 

 from rotation about a vertical axis to rotation about a horizontal axis. 

 This arrangement served later as a model for the tachometer designed 

 to measure our vertical speed in the course of dives : we shall speak of 

 it again on page 97. The motors were designed and built by the 

 Ercole Marelli Company of Milan : the reduction gears were built by 

 the Navalmeccanica of Castellammare di Stabia. We gave up the idea 

 of furnishing the bathyscaphe with a rudder which, in view of the low 



[37] 



