Thus if the temperature varies, the pressure alters on each side of 

 the mercury to the same extent without influencing its position; but 

 if the quantity of oxygen in the cabin increases or diminishes, the pres- 

 sure in the cabin varies and the mercury indicates it. 



Thus the control of the apparatus for the regeneration of the air 

 is very simple. It is set going as soon as the Siemens apparatus indicates 

 1% of carbon dioxide and this is done either by the oxygen injector 

 or by the ventilator, so as to maintain the two 

 columns of mercury at the same level. If the 

 mercury rises in the closed side of the U-shaped 

 tube, the ventilator is set going: in the inverse 

 case, the oxygen injector is put into operation. 



It would be easy to take into account the con- 

 centration of carbon dioxide to make corrections. 

 But such precision is superfluous. 



PRESSURE GAUGES 



Although the pressure gauges were not vitally 

 important to us, it was essential, nevertheless, 

 for them to be in good working order, since they 

 indicate and record our depth. In the balloon we 

 also had pressure gauges, but while these had 

 to measure variations in pressure which never 

 reached one atmosphere, those in the bathy- 

 scaphe had graduations reaching up to 600 

 atmospheres, which corresponds to a depth of 

 3J miles in fresh water, and slightly less in sea 

 water.l 



The principle of the high-pressure gauge is 

 very simple : the interior of a steel tube, bent into 

 a semi-circle, the Bourdon tube, is subjected to 

 the pressure that is to be measured, in our case the pressure of sea 

 water. If the pressure increases, the tube straightens slightly, and this 

 movement is transmitted to the needle of the gauge. 



The Trieste has four pressure gauges installed : a large one recording 



^ At the time of the construction of the FNRS 2 our pressure gauges were 

 supplied by the firm of Haenni (Jegenstorf, Switzerland) and after these had 

 been handed over to the French Navy (where they were used on the FNRS 3) 

 the same firm constructed a set of improved apparatus for us. 



[95] 



Fig. II. Apparatus 

 controlling the 

 density of air in 

 the cabin 



