811,853. Trawl fishing. RAS, M. D. C. Nov. 22, 1955 [Dec. 7, 1954], No. 33382/55. Class 48. [Also in Groups XXIX and XXXVII] A variable buoyancy float structure F adapted to be towed at variable depth and for towing a trawl net N comprises at least one tank 1, 2, Fig. 5, charged with pressure air, at least one chamber 3 having ports 6, 7 through which seawater may enter or discharge, means for the controlled admission and escape of compressed air for variation of the amount of seawater in the chamber and accordingly for adjustment of the buoyancy of the float and a compressed air container 13 carried by the float and supplying the air admission and escape means, the latter comprising an air delivery valve 17, Fig. 6, and an air discharge valve 19 resiliently loaded towards closed position, the two valves being connected for alternate opening and closing by motivating means consisting of a member 73 displaceable by a diaphragm 15 subjected on one side to the pressure within the chamber 3 and on the other side to air at a pressure adapted to be varied in accordance with the desired depth of submergence, and a self-closing valve 32, Fig. 11, controlling the supply of pressure air to the side of the diaphragm 15 subjected to the variable pressure and a selfopening valve 31 controlling the escape of air from the same side of the diaphragm 15, these two valves being interconnected for alternate opening and closing by an armature 35 of an electric solenoid 36 energizable by a controller remote from the float, for instance on the trawler. The float comprises a cylindrical drum of mild or stainless steel or brass having partitions 4, 5 forming the pressure air tanks 1, 2 and the central chamber 3. The ports 6, 7 for the ingress and egress of seawater from the central chamber 3 are remotely spaced and are controlled for alternate opening and closing by valves 8, 9 actuated by a common rod 10 connected to a pivoted weighted lever 11, the arrangement being that when the float is tilted the port at the lower level is open and the other closed. The amount of seawater in the chamber 3 is adjusted so that the float is in equilibrium and the float is caused to rise or sink to a new desired depth by blowing seawater out or allowing it to enter through the ports 6, 7. The equilibrium is again restored at the new depth by blowing out the seawater which was allowed to enter or vice versa. The means for controlling the admission and escape of compressed air from the chamber 3 to vary the amount of seawater therein consists of an electromagnetic mechanism empr, Fig. 11, which provides an air pressure in a chamber 30 slightly greater than the ambient pressure at the depth to which it is desired to move the float, and a balancing mechanism bm, Fig. 6, which responds to the pressure in the chamber 30 of the electromagnetic mechanism empr to control both the air delivery valve 17 and the air-discharge valve 19. The balancing mechanism, Fig. 6, is suspended from a closure 60 in the top of the chamber 3 and comprises a balance chamber 14<SP>1</SP> closed by the diaphragm 15 and connected by a pipe 16 to the chamber 30 of the electromagnetic mechanism empr. The upper surface of the diaphragm 15 is subjected to the pressure in the chamber 3 and to the action of a spring 21 disposed between the diaphragm and an apertured cover 76. The air delivery valve 17 consists of a jet supplied from the pressure air tanks 1, 2 by a line 18 and adapted to be closed by being urged into contact with a gasket 17<SP>1</SP> on the lower end of the stem 22 of the air-discharge valve 19. The latter comprises a gasket 19<SP>1</SP> urged by a spring 20 to close a port in the closure 60. If the pressure in the balance chamber 14<SP>1</SP> is increased the diaphragm pushes the stem 22 upwardly to open the air-discharge valve 19 so that air escapes from the central chamber 3 and seawater enters through one of the ports 6, 7. The float therefore sinks and consequently the pressure in the central chamber 3 increases so that the diaphragm 15 eventually allows the air discharge valve 19 to close. The float however continues to sink and the diaphragm eventually opens the air jet 17, whereupon air at high pressure enters the central chamber and drives out the seawater. The float therefore rises and the decreasing pressure in the central chamber 3 causes the diaphragm 15 to close the air jet 17 and later to open the air-discharge valve 19. Seawater again enters and the float sinks. The float thus oscillates vertically but the oscillations die out and the float finally rests with both valves 17, 19 closed at a new depth corresponding to the new pressure in the balance chamber 14<SP>1</SP>. When it is required to bring the float to the surface the air pressure in the balance chamber 14<SP>1</SP> is reduced so that all the seawater is blown out of the central chamber 3. The air jet 17 is then closed by the action of a system of two differential diaphragms 24, 26 which cover the bottom of the balance chamber 14<SP>1</SP> and are connected together by a short shaft 25 which engages a push-rod 29 secured to the diaphragm 15. The space 27 between the differential diaphragms 24, 26 is vented to the outside of the upper part of the float. When the seawater is completely expelled, thrust on the larger differential diaphragm 26 causes the short shaft 25 to push the rod 29 and the diaphragm 15 upwardly to close the air jet 17. The electromagnetic mechanism empr, Fig. 11, is supported inside the central chamber 3 and comprises the chamber 30 connected to the balance chamber 14<SP>1</SP> of the balancing mechanism bm and the self-closing valve 32 and the self-opening valve 31 which are adapted to supply air to and bleed air from the chamber 30. The self-closing valve 32 is connected to the pressure air line 18 and the self-opening valve 31 is connected to a helically arranged tube 30<SP>1</SP> leading to the central chamber 3. Both valves 31, 32 are operated by a platform 34 supported by the armature 35 of the solenoid 36. In an intermediate position of the platform both valves are closed, in a raised position of the platform the compressed air supply valve 32 is open and the bleed valve 31 is closed and in a lowered position of the platform the compressed air supply valve 32 is closed and the bleed valve 31 open. The armature 35 is also connected to a diaphragm 58 sealing air at low pressure in a chamber 58<SP>1</SP>. If the current in the solenoid is increased by the controller on the trawler the armature moves upwardly to open the air supply valve 32 and the resulting increased pressure in the chamber 30 acts on the diaphragm 58 to return the armature to its original position with the air supply valve 32 again closed. If the solenoid current is reduced the diaphragm 58 moves the armature downwardly to open the bleed valve 31. The resulting reduced pressure acting on the diaphragm enables the solenoid to raise the armature to its original position with the bleed valve again closed. The chamber 30 is provided with a safety valve 33. The pressure of the air from the containers 13 is reduced by a device casr, Fig. 15, comprising a valve 39 secured to a diaphragm 41 urged to close the valve by an adjustable spring 66. The space 38 below the diaphragm 41 is connected to the air line 18 supplying the valve 32 of the electromagnetic mechanism empr and the air jet 17 in the balancing mechanism bm. The space 40 above the diaphragm 41 is connected by a line 42 to the central chamber 3 so that if the diaphragm 41 leaks air passes into the central chamber and causes the float to rise. The supply line 18 is provided with a safety valve 29<SP>1</SP>, Fig. 5. The electric supply to the solenoid 35 is controlled by a safety switch device sps, Fig. 15, operated by the pressure in the container 13 and adapted to de-energize the solenoid and thus cause the float to rise if the pressure in the containers drops below a safe value commensurate with the depth at which the float is operating. The device sps comprises a diaphragm 43 subjected to the pressure in the containers 13 and carrying a contact 44 normally held against two fixed contacts 45, 46 in the electric supply line to the solenoid 35. A further diaphragm 50 is subjected to the pressure in the central chamber 3 by a pipe 42<SP>1</SP> and carries a plunger 55 which is adapted to push the contact 44 away from the contacts 45, 46 when the air pressure in the containers is below the safe level. An adjustable spring 53 bears on the diaphragm 50. The net N, Fig. 2, has a circular mouth defined by a steel ring N<SP>1</SP> formed in segments which are attached to each other by spigot and socket joints locked by cross-cotters. The cod end is made of tarred twine and the net body of nylon. The net is rendered buoyant by a steel float N<SP>11</SP> which is permanently charged with compressed air. The variable buoyancy float F is arranged in front of the net mouth and is attached thereto by a hemp rope tl. The steel tow line TL from the trawler is attached to the float F. An electric field for controlling the movements of the fish or to electrocute or stun them is created at the mouth of the net between an extended cathode provided by the variable buoyancy float F and the steel tow line TL and a graphite anode A attached to the hemp TL and a graphite anode A attached to the hemp rope tl and disposed in front of the net mouth. A graphite sub-anode A<SP>1</SP> may be mounted inside the net and in this case the anode A is connected to the net ring N<SP>1</SP> through stainless steel wires 79. The diameter of the electric field is altered by varying the length of the hemp rope tl. The electric supply to the solenoid in the float F and for the electric field is carried by a two-core rubber insulated cable C having a steel wire core. The steel towing wire TL serves as the return lead for the solenoid and electric field. The p