685,348. Gas turbine plant. SULZER FRERES SOC. ANON. Feb. 5, 1951 [Feb. 6, 1950], No. 2800/51. Class 110(iii) [Also in Group XXIX] The invention relates to a gas turbine plant of the kind in which at least two turbines are arranged in parallel, one of which drives a compressor whilst another supplies useful power and in which a pressure sensitive device controls the pressure level of the working medium in the plant by regulating the distribution of the working medium between the turbines. The invention consists in influencing the pressure level adjusting device by the speed of the compressor so that when the speed of the compressor increases, the pressure level adjusting device tends temporarily to reduce the amount of working fluid passing to the compressor driving turbine and vice versa. The invention is described with reference to a gas turbine plant, Fig. 1, in which air flows through a closed circuit comprising low and high pressure compressors 1, 3, a heat exchanger 5 in which the air is heated by the exhaust gases of the compressor-driving turbine 13, a heater 10 in which the air is further heated by fuel burnt in a combustion chamber 20 and the turbine 13 the exhaust of which passes back to the compressor 1 through the heat exchanger 5. Air extracted from the closed circuit is supplied to the combustion chamber 20 and the combustion products after passing through the heater 10 are used in a useful power turbine 22. The air extracted from the circuit is replaced by a compressor 23. To adjust the distribution of the working fluid between the turbines 13, 22, the turbine 22 has a by-pass pipe 29 fitted with a regulating valve 30. The valve 30 is controlled by a pressure level adjusting device 33 which is responsive to the pressure in the conduit 12 leading to the inlet of the turbine 13 and which has a lever 35 to enable this pressure to be preselected. Closing of the valve 30 causes less air to be withdrawn from the circuit and more air to flow through the compressordriving turbine 13. This causes an increase of the pressure in the circuit which results in an increase in the pressure drop across the useful output turbine 22 and, it is stated, increases the output of this turbine. The turbine 13 is also provided with a by-pass pipe 36 fitted with a regulating valve 37. The valve 37 is also operated by the pressure level adjusting device 33 in the opposite sense to the movement of the regulating valve 30. The pressure level adjusting device 33 is shown in detail in Fig. 2. Any change of pressure in the conduit 12 causes the left hand end 41 of the lever 42 to move and operate a control valve 49 which allows pressure fluid to flow to one side or the other of a piston 53 which moves and stresses or relieves a spring 56 by which it is connected to a pressure regulating valve 57. As the stress in this spring is balanced by the pressure in the chamber 63, changes in the stressing of the spring will allow the valve 57 to move and allow pressure fluid to flow into the chamber through the pipe 61 or out of the chamber through pipe 62 until the balance is restored. The piston 53 is connected to the floating cylinder 55 of yielding return device, the piston 58 of which is connected to the right hand end 43 of the lever 42. A pipe 60 having a throttle 59 connects the two ends of the cylinder 55. Movement of the piston 53 thus causes the control, valve 49 to return to its neutral position. The pressure in the chamber 63 is transmitted through pipes 32, 38 to the servomotor valves 30, 37. The arrangement is such that an increase in pressure in the pipes 32, 38 first closes the valve 30 and when this valve is fully closed commences to open the valve 37. The end 43 of the lever 42 is also attached through a spring 46 to a cam follower 45 which co-acts with a cam 44 attached to the pressure setting lever 35. Movement of this lever causes the end 43 of the lever to move and the control valve 49 to allow pressure fluid to be supplied to one side or other of the piston 53 which in turn operates the valve 57 as above described. The control finally comes to rest when the spring 46 is unstressed which is effected by the operation of the yielding return device. When the control is in a state of equilibrium, the spring 46 is unstressed and the control valve 49 in its neutral position, so that the pressure in the conduit 12, as controlled by the capsule 40, corresponds to that selected by the lever 35. When the regulating valve 37 is first operated, the pressure level in the plant is influenced temporarily in the wrong sense. To avoid unstability of the pressure level adjusting device due to this cause, centrifugal governor 77 driven from the shaft of the compressor-driving turbine 3 is coupled through a linkage 79, 80, a floating liquid-filled cylinder 82 and linkage 85, 86, 87 to an axially slidable sleeve 88 disposed around the control valve 49. Any change of speed of the turbine 13 causes movement of the sleeve 88 in the opposite direction to that in which the valve 49 is at first moved by the capsule 40 in response to a corresponding change in pressure level. If the output selecting lever is moved far enough in the increase power direction the valve 37 is opened as described and the speed of the turbine increased. This speed increase causes the sleeve 88 to move upwards which gives an equivalent effect on the control valve 49 of an increase in pressure in the conduit 12. Since the instability referred to above is only temporary, a neutralising device consisting of springs 90 and 91 and the floating cylinder 82 is provided to allow the sleeve 88 of the control valve 49 to return slowly to its neutral position. The application of the invention to an open cycle gas turbine plant is also described. In this arrangement only the compressor driving turbine is provided with a bypass pipe and regulating valve. The regulating valve is operated through levers by a pressure level adjusting device similar to that described with reference to Fig. 2 except that the pressure responsive capsule 40 and the pressure setting lever 35 act on the same end of the lever 42 and the yielding return device acts on the other end of the other end. Another form of stability control is also described. The centrifugal governor 77 also acts on the regulating valve by means of servo-motor system which incorporates a yielding return device so that the effect of changes of speed of the governor on the regulating valve are temporary. Reference has been directed by the Comptroller to Specifications 597.337 and 620,748-