the invention will be described with reference to the embodiment shown in fig2 in which those components which have been described with reference to fig1 have been similarly designated . in a high pressure turbine 30 supplied with high pressure steam and a low pressure turbine 40 supplied with low pressure steam , according to the invention at least their steam chambers are independent from each other . furthermore , according to the invention , the high pressure turbine 30 is connected to a steam condenser 31 at a low vacuum while the low pressure turbine 30 is connected to a steam condenser 41 at a high vacuum . in this case , each of the steam condensers 31 and 41 is formed as a jet condenser . the steam condensers 31 and 41 are provided with condensation pipes 32 and 42 , condensation pumps 33 and 43 , and cooling water pipes 34 and 44 , respectively . in accordance with one aspect of the invention , these elements are connected in series with one another . the steam condensers 31 and 41 are further provided with gas extracting pipes 35 and 45 and gas extractors 36 and 46 , respectively . in a geothermal turbine installation constructed as described above according to the invention , the high pressure steam from the steam separator 3 is fed through the high pressure steam pipe 4 , the inlet valve 12 and the inlet 10 into the high pressure turbine 30 where it is expanded to perform work . thereafter the steam is introduced into the steam condenser 31 where it condenses into water . in this case , the high pressure steam from the steam separator 3 is primarily saturated vapor including a large quantity of non - condensate gas . accordingly , in order to prevent erosion of the blades in the low pressure stage of the high pressure turbine 30 and to maintain a practical and economical capacity of the gas extractor 36 , the degree of vacuum of the steam condenser 31 is set low for instance 0 . 11 ata or lower as in the case of the conventional steam condenser 14 in fig1 . on the other hand , the flash steam from the hot water flash tank 6 is introduced through the low pressure steam pipe 7 , the inlet valve 13 and the inlet 11 into the low pressure turbine 40 where it expands to perform work . thereafter , the steam is introduced into the steam condenser 41 at a high vacuum where it condenses into water . in this case , the flash steam from the hot water flash tank 6 has a high entropy and includes only a small amount of non - condensate gas . accordingly , the degree of vacuum of the steam condenser 41 is set higher , for instance 0 . 08 ata , than that of the steam condenser 31 of the high pressure turbine 30 in correspondence with the conditions of the flash steam . in this case , the cooling water pipe 44 of the steam condenser 41 at a high vacuum and the cooling water pipe 34 of the steam condenser 31 at a low vacuum are connected in series with each other . the cooling water is thus fed through the pipe 44 into the steam condenser 41 of the low pressure turbine 40 where it is heated . the water thus heated and the condensate from the discharged steam from the low pressure turbine 40 are subjected to a pressure boost and are supplied through the pipes 42 and 34 to the steam condenser 31 of the high pressure turbine 30 . thus , with the flow rate of the cooling water substantially equal to that of the conventional geothermal turbine installation shown in fig1 a high vacuum is satisfactorily provided in the steam condenser 41 of the low pressure turbine 40 , while low vacuum is satisfactorily provided in the steam condenser 31 of the high pressure turbine 30 . the high pressure turbine 30 and the low pressure turbine 40 can be constructed as a double flow - type turbine using a common casing . furthermore , the steam condensers 31 and 41 can be formed as a single unit as a so - called &# 34 ; multiple - stage steam condenser &# 34 ;. as the content of non - condensate gas in the discharge steam of the low pressure turbine 30 is very low , the power required for operating the gas extractor 46 can be very low . especially , if the outlet port of the gas extractor 46 is connected to the suction port of the gas extractor 36 of the steam condenser 31 of the high pressure turbine 30 , then the gas extractor 46 can be operated with a small amount of power because the pressure of the gas extractor 46 is boosted to the internal pressure of the steam condenser 31 of the high pressure turbine 30 . any increase in the cost of equipment can be disregarded because as a practical matter such cost is substantially offset by the improvement of the energy conversion efficiency of low pressure steam . as is clear from the above description , in a geothermal turbine installation according to the invention , with minimum expenditures for equipment and operation , the high pressure steam from a geothermal well and the flashed steam from the hot water are introduced into turbines having separate steam chambers and the degree of vacuum of the steam condenser of the low pressure turbine supplied with flash steam can be made higher than the degree of vacuum of the steam condenser of the high pressure turbine supplied with high pressure steam whereby the energy conversion efficiency of an overall plant including such a geothermal turbine installation can be increased and the output power can be increased by 4 to 5 %. in the embodiment shown in fig2 the steam and water mixture , namely the geothermal steam , is subjected to separation at the steam separator . however , in the case of a geothermal well from which a large quantity of hot water is discharged , instead of a steam separator , a flash separator can be provided . the steam condensers 31 and 41 may be surface contact - type steam condensers instead of direct contact - type jet condensers . furthermore , the condensation ( pressure increase ) pump 43 connected to the condensation pipe 42 of the low pressure turbine 40 can be omitted as the case may be . fig3 shows an example of a turbine according to the invention . in this turbine , the high pressure steam performs work while passing through the left turbine expansion section and is introduced through the left discharge port into the steam condenser on the low vacuum side . the flashed low pressure steam performs work while passing through the right turbine expansion section and is introduced through the right descharge port into the steam condenser on the high vacuum side . this example is of a one - cylinder double flow exhaust type . however , it is obvious that the turbine can as well be a multi - cylinder multi - flow exhaust type . this embodiment of multiple - stage pressure steam condensers produces excellent results as described above . however , the extent of the improvement obtained thereby is affected by the ratio of flow rates of the high pressure steam and the low pressure steam . fig4 is a graphical representation showing the effects of varying the ratio of flow rate . the vertical axis indicates the incremental increase in percent (%) of a turbine output with multiple - stage pressure steam condenser employed as described above with respect to a turbine output obtained when the discharged high pressure steam and low pressure steam are introduced into a single steam condenser . the horizontal axis indicates a low pressure steam flow rate ratio , specifically , the percentage of a value which is obtained from the following formula : ## equ1 ## where g1 is the flow rate of high pressure steam and g2 is the flow rate of low pressure steam . as is apparent from fig4 the increase of the output value is maximum when the ratio of flow rate between the high pressure steam and the low pressure steam is about 1 : 1 . as the ratio becomes larger or smaller than 1 : 1 , the favorable effects of using the multiple - stage pressure steam condenser decrease . in actual geothermal steam , the quantity of flashed low pressure steam can be less than that of high pressure steam so that employment of a multiple - stage steam condenser is not satisfactorily beneficial . in view of the foregoing , the invention is further intended to provide a geothermal turbine installation which can produce its maximum beneficial effect even if the flow rate of high pressure steam is different from that of low pressure steam . second and third embodiments of the invention will next be described in conjunction with fig5 and 7 . in fig5 and 7 , these components which have been described with reference to fig1 are similarly numbered and a detailed description of them will be omitted . fig5 shows a second embodiment of the invention in which a turbine 50 has a low pressure turbine section 31 and a high pressure turbine section 32 . the high pressure turbine section is divided into two parts , a first part 33 and a second part 34 . the first part 33 connects the low pressure turbine section 31 with the steam chamber forming an ordinary mixed - pressure turbine . the turbine shaft of the second part 34 is common with that of the first part . however , the steam chamber thereof is independent of that of the first part . unlike the embodiment shown in fig2 the inlet 14 of high pressure steam is provided between the first and second parts of the high pressure turbine where the high pressure steam is distributed so that a part of the high pressure steam is supplied to the first part 33 and the remaining steam is supplied to the second part 34 . the high pressure steam expanded in the first part is mixed with low pressure steam supplied from a low pressure steam inlet 16 and the steam mixture is continuously expanded in the low pressure turbine section . the discharged steam is introduced into a steam condenser 17 of high vacuum . on the other hand , the steam supplied to the second part 34 of the high pressure turbine in expanded only in the second part and the discharged steam is introduced into a steam conductor 18 of low vacuum . the distribution of high pressure steam at the inlet 14 is determined so that , according to the output increment characteristic curve shown in fig4 the ratio of the quantity of discharged steam from the low pressure turbine section 31 to the quantity of discharged steam from the second part 34 of the high pressure turbine section approaches 1 : 1 as nearly as possible . this may be achieved by employing a method in which the distribution of high pressure steam is effected as calculated in advance in compliance with the ratio of flow rate between the high pressure steam and the low steam pressure in the geothermal steam which is supplied from a particular steam well . alternatively , an arrangement may be provided so that the distribution can be controlled as desired so as to be applicable to any steam well . fig6 shows an example of the turbine constructed according to the above - described embodiment of the invention . a larger part of the high pressure steam performs work while passing through the left turbine expansion section and is introduced through the left turbine discharge port into the steam condenser on the low vacuum side . the remaining part of the high pressure steam performs work while passing through the left high pressure turbine expansion section then is mixed with low pressure steam and is introduced through the right turbine discharge port into the steam condenser on the high vacuum side . this example is of a turbine of the one - cylinder and double flow exhaust type . however , it may readily be modified to one of the multicylinder multi - flow exhaust type . the third embodiment of the invention is as shown in fig7 . similar to the embodiment shown in fig2 a turbine 40 has a low pressure turbine section 41 and a high pressure turbine section 42 whose steam chambers are independent of each other . however , in this embodiment a part of the high pressure turbine section is connected through a communication path 43 to the low pressure turbine 41 . a part of the high pressure steam expanded in the high pressure turbine section 42 is extracted and is supplied together with low pressure steam from an inlet 16 to the low pressure turbine section 41 . the quantity of steam extracted may be a predetermined constant quantity or it may be varied by means of a valve provided in the communication path 43 . at any rate , it is preferable that the ratio of the quantity of discharged steam from the high pressure turbine section 41 to the quantity of discharged steam from the low pressure turbine section 41 be as close to 1 : 1 as possible . fig8 is a cross - sectional view showing an example of the turbine according to the third embodiment of the invention . first , the high pressure steam performs work while passing through the high pressure expansion section . thereafter , a larger part of the high pressure steam performs work while passing through the left expansion section and is introduced through the left discharge port into the steam condenser on the low vacuum side . the flow direction of the remaining part of the high pressure steam is changed to the right and that part is introduced through the turbine steam chamber to the right turbine expansion section where it is mixed with low pressure steam . the steam mixture performs work while passing through the right expansion section and is introduced through the right discharge port to the steam condenser on the high vacuum side . this example of a turbine is one of a single - cylinder and double flow exhaust type . however , it may be modified if desired to be a multi - cylinder multi - flow exhaust type . in the second and third embodiments of the invention , a large part of the high pressure steam containing a large quantity of non - condensate gas is condensed into water by the steam condenser 18 ( fig5 and 7 ) on the low vacuum side and therefore the power required for the gas extractor is not excessive and the above - described difficulty in which high wet is formed by excessively high vacuum is eliminated . furthermore , the discharged steam from the low pressure turbine section , a large part of which is steam obtained by flashing , is condensed into water by the steam condenser 17 of high vacuum . therefore , the energy of the low pressure steam can be effectively converted into hynetic energy . in addition , it is possible to let the ratio of the flow rate of discharged steam from the high pressure turbine section to the flow rate of discharged steam from the low pressure turbine section approach 1 : 1 and therefore the positive increment in output power increment can be fully realized .