These pressure gauges, graduated to record from o to 3J miles in 

 depth, naturally cannot possess great sensitivity. Now, in certain 

 cases, above all if the bathyscaphe is near to the surface, a much 

 greater sensitivity would be desirable. Thus for the FNRS 2 I invented 

 and had built a pressure gauge in which the height of a column of 

 mercury varies by 3 ft. for depths changing from o to 3 J miles. It ' 

 was thus delicate enough to render perceptible variations in depth 

 of 3 ft., the displacement of the mercury being proportional to the 

 pressure. This pressure gauge was given with the FNRS 2 to the 

 French Navy by the Belgian National Fund.l As the construction of 

 this gauge is very costly and would have demanded a good deal of my 

 time, I gave up the idea of reproducing it. Instead I designed a small 

 gauge, very simple and quite sensitive, graduated from o to 330 ft. 

 depth. The description of this is given in the Appendix. 



THE TACHOMETER 



The pressure gauges certainly showed us the depth that we had 

 reached, but that was not enough. For piloting purposes it is 

 important to be able to determine at any moment what is our vertical 

 speed. According to the circumstances, this speed determines the 

 need to drop ballast or petrol. It is true that the slope of the line drawn 

 by the recording gauge allowed us to determine this speed, but only 

 after a considerable delay, since the graduated paper moves only at 

 the rate of xr in. a minute. Several minutes are then necessary for 

 the slope of the recorded line to be observed. This is too long. 



Exactly the same problem occurs with the free balloon and two 

 instruments have been constructed to measure its vertical speed — the 

 variometer and the vane anemometer or wind gauge. 



The variometer cannot be utilized in a bathyscaphe. But the vane 

 anemometer can be employed almost as it stands (Fig. 12). It consists 

 of a bladed fan h mounted with its vertical axis of rotation. The vertical 

 wind produced by the motion of the balloon starts it rotating and 

 this speed of rotation is proportional to the speed of the balloon. In 

 the case of the bathyscaphe a slight difficulty arises : if one wishes to 

 observe the vanes directly, the anemometer must be placed beneath 

 the float, in the neighbourhood of the porthole ; but in this position 



1 I am not sure that it is still used, the principle varying too much from the 

 norm. 



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