o-ii atmospheres. This pressure is insignificant from the viewpoint 

 of the strength of the hull, but it exceeds the hydrostatic pressure 

 which may be induced by the difference between the density of water 

 and of petrol over a height of 11-55 feet (the height of the compart- 

 ments). It is therefore impossible that as a result of a hole being made 

 in the upper part of a compartment the petrol escaping will suck 

 up the water through L. The petrol will not be able to leave the 

 float unless the water comes in by the same opening. This is of first 

 importance, for two reasons. First of all, there is no difference of 

 pressure which will induce the petrol to gush out by a large vent. 

 The outflow will then take place more slowly (the water and the petrol 

 having to pass each other in the same opening) than if the water, 

 entering at the bottom of the float, produced a hydrostatic pressure in 

 the upper level of the float. If the damage to the upper part is produced 

 at the surface — for example by collision (always to be feared) — the 

 time thus gained could be very valuable. It could even, in certain 

 circumstances, avoid catastrophe. On the other hand, if it is only a 

 question of a slight leak, a local fissure in the welding for example, 

 the petrol, pushed by the water entering directly at the base of the float, 

 at the expiry of a certain time, would escape in considerable quantities. 

 But if the water can only enter by the same fissure, it must meet and 

 pass the petrol. In a fissure of small dimensions, this passing would 

 not take place and the loss of petrol would be avoided. 



Let us now see the role of the upper flap-valve in our double valve. 

 If the petrol expands, it acts upon the water which is in the bottom of 

 compartment No. 7. This water then rises into the annular space 

 between the two tubes (see Fig. 8), lifts the upper flap- valve, then 

 through the central tube escapes to the sea. The object of the flap- valve 

 is solely to prevent the water coming in by this same route due to 

 suction caused by a leak. 



The double valve is placed on deck so that it will be easy at any 

 time to check or repair it. It is covered by a large dome hermetically 

 closed which protects it mechanically and removes all possibility of air 

 entering in case of loss from the upper valve. 



Let us now look at a few constructional details of our valve. 



To begin with, the flap-valves must be made to move vertically so 

 as to avoid all danger of seizing. If any rod slides in a bearing it is 

 always to be feared that it may be blocked by a foreign body — a little 

 grain of sand for example. To avoid an accident of this sort, I deviated 



[ 177 ] 



