referring now to the drawings and , first , particularly to fig1 thereof , there is diagrammatically shown therein a delivery system 1 of a sheet - fed printing press . therein , paper sheets 4 gripped at leading ends thereof by gripper bars 2 are fed from a non - illustrated last printing unit of the printing press in a direction towards a delivery pile 6 via a so - called guide body or guide member 3 . as is apparent from the enlarged view of fig2 the guide member 3 is embodied as a box having a hollow interior into which a blower 7 blows air via a connection line 8 , thereby generating an overpressure p v , which escapes via a multiplicity of nozzle openings 5 and thus builds up an air cushion between the respective guide member 3 and the surface thereof , on the one hand , and the underside of the sheet 4 guided thereabove , on the other hand . in the plot diagram or graph of fig3 by way of example , characteristic curves of the two essential components of an air cushion guide are shown , namely that for a typical air generator , i . e ., a blower , represented by curve b , and that of a consumer , i . e ., a number of nozzle openings in a guide member , represented by curve a . in each case , the volumetric flow q generated by the generator , and the volumetric flow through the consumer , respectively , are plotted as functions of the pressure p v , i . e ., the pressure at the consumer acting as a throttle , and of the pressure difference from the generator at the consumer because of the throttling action . the characteristic curve of the consumer having the nozzle openings acting as a throttle ( curve a ) begins at the zero point and then rises . the characteristic curve of the air generator ( curve b ), conversely , attains the maximal pressure thereof when the volumetric flow , because of the throttling action of the consumer , tends toward zero , i . e ., in the case wherein all of the nozzle openings are closed . the resultant operating point p 1 of the air cushion guide takes the form of an intersection of the characteristic curves b of the air generator and a of the air consumer . if the power of the generator is varied , for example , by varying the rotary speed of the blower , by throttling in the feed line to the consumer , or by a bypass around the consumer line , then the generator characteristic curve does , in fact , change , as indicated by the curves b ′ and b ″ shown in broken lines in fig1 . the intersections with the unchanged consumer characteristic curve become operating points for the air cushion characteristic curve , however . accordingly , by varying the power of the blower , as represented by the curve a shown in a solid or unbroken line in fig1 the volumetric flow can be varied approximately proportionally to the pressure applied at the nozzles . because the flow speed , in turn , depends upon the pressure , as represented by the plot diagram or graph in fig4 when the consumer characteristic remains the same , the flow speed cannot consequently be adjusted independently of the volumetric flow by varying the power of the blower . this has the consequences for the flotation height or level of the sheet guided via the air cushion guide , which were mentioned hereinbefore in the introduction hereto . the overpressure present at the nozzle openings of the guide member is converted into flow speed , assuming a loss - free flow , in accordance with the following equation : c = 2 × ( κ κ - 1 ) × ( p v p v ) × ( 1 - p 0 ( κ - 1 κ ) p v ) + c 0 2 ( 1 ) the accelerated air is propagated uniformly , due to the shape of the nozzles , between the guide member that includes the nozzle openings and the sheet guided thereabove . in accordance with the continuity equation according to which the volumetric flow through the nozzle openings , because of the chamber pressure p v applied thereat , is equivalent to the volumetric flow q 2 under the sheet , there results , for a vertical cross section of the flow under the sheet , h = q b × c . ( 4 ) ρ v = density of the air at the initial or supply pressure of the chamber p o = pressure of the air flow under the sheet ( equivalent to atmospheric pressure ) from this dependency , it is apparent that the flotation height or level of an air cushion guide can be varied only whenever the quotient of the volumetric flow and the flow speed is varied , or in other words if the volumetric flow is increased but the flow speed is not simultaneously increased as well , or if the volumetric flow is increased markedly disproportionately relative to the flow speed . because , as noted hereinbefore , the flow speed is dependent upon the pressure p v at the nozzle openings , the pressure should accordingly be variable independently , or in other words not in proportion to the volumetric flow through the nozzles , in order to achieve the objective of the invention . in accordance with the exemplary embodiment of the invention shown in fig5 this is made possible in that the number of active nozzle openings covered by the guide material is variable . to this end , the chamber 13 disposed under the guide baffle 13 b and together therewith forming the guide member of an air cushion guide is provided with a plurality of individual subchambers 16 a , b , and so forth , having air supply lines with electrically triggerable valves , respectively , connected thereto at the underside thereof . the air supply lines which discharge into the bottom plate 13 a of the guide member are identified by reference characters 14 a , b and c , for example , and the valves associated therewith are identified as 15 a , b and c , respectively . respective groups of nozzles are associated with the corresponding chambers 16 a , b and c of the guide member in the guide baffle , each of the nozzle groups , in the interest of greater clarity , being represented symbolically by only a single nozzle 12 a to c , respectively . the chambers 16 a , 16 c are supplied jointly by a variable - rpm blower 17 . the blower 17 , like the valves 15 a , 15 b , 15 c , and so forth , is connected to an electronic control unit 19 , which is supplied with a desired or setpoint value of the flotation level or height h soll of the contact - freely guided sheet by the central control computer of the printing press to which the delivery system shown in fig1 belongs . if the flotation height is to be changed to lower values , the control unit 19 then turns off some of the valves 15 a to c , and so forth , thus reducing the number of active nozzle openings . in this way , the volumetric flow of gas blown into the air cushion is reduced , while at the same time , because of the higher throttling action of the nozzle array , the pressure generated by the blower 17 rises , and thus the flow speed of the airstream emerging from the nozzles 12 rises as well . consequently , an operating point for the air cushion guide is obtained at the point marked p 2 in the graph of fig3 . this corresponds to the point of intersection of a somewhat flatter consumer characteristic curve a ′ with the unchanged generator characteristic curve b . if the flow speed under the sheet is to be adapted or matched simultaneously to the previous value , then the blower is regulated additionally to a lesser rpm , so that the generator characteristic curve b ″ and thus an operating point at the location marked p 3 in the graph of fig2 results . it is clear that the guide member must be divided into individual subchambers 16 a , b , c , and so forth in such a manner that no disturbing inhomogeneities in the flotation height or level of the sheet are produced . whereas , in the preceding exemplary embodiment according to fig5 individual groups of nozzles , and thus in each case a discrete number of nozzle openings in the nozzle guide baffle , are supplied with blown or blast air and consequently switched to “ active ”, in the next exemplary embodiment according to fig6 a change in the number of active nozzle openings is accomplished by an infinitely variable gear . features or elements therein which are identical to those shown in fig6 are provided in fig5 with reference numerals or characters which have been increased by 100 and will not be explained again . in fig6 the chamber 113 of the guide member is subdivided by two inserted ribs 115 a and 115 b into three sections 116 a , b , c , of which the middle section 116 c communicates with a blower 117 via a hose line 118 . the two ribs 115 a and 115 b can be adjusted oppositely to one another with the aid of threaded spindles 114 a and 114 b , a motor 120 connected to the control unit 119 , and a toothed belt 110 . in this manner , the size of the two chambers 116 a and 116 c , which are not connected to the blower 117 , can be varied , thereby deactivating the nozzles associated therewith and located thereabove . in order to increase the volumetric flow and thus the flotation level or height of the sheet fed above the guide baffle 113 b , the chambers 116 a and 116 c are accordingly made smaller by displacement of the ribs 115 a and 115 b , and contrarily , in order to decrease the volumetric flow and the flotation height of the respective sheet , the chambers 116 a and 116 c are made larger by suitably displacing the ribs 115 a and 115 b . in the next exemplary embodiment shown in fig7 a and b , the volumetric flow of the air cushion guide can be varied independently of the flow speed , because the cross sections of individual nozzle openings or groups of nozzle openings , or of all the nozzle openings , are variable . to that end , nozzle openings are formed in a conventional manner by an embossing / stamping process in the guide baffle 213 b above which the sheet 201 floats , the embossing / stamping process forming tongues 212 a , b , c , and so forth protruding inwardly into the guide member and directing the airstream of the overpressure air in the chamber 213 of the guide member so that it can reach underneath the guided sheet 201 . the underside of the tongues 212 a , b , c , and so forth , which for example are formed of sheet steel , have metal strips 214 a of some other material , such as aluminum , adhesively secured thereto , so that , in the region of the movable tongues 212 of the nozzles , a bimetallic property is produced . at the same time , a heating coil 216 of a controllable heater 215 is accommodated in a supply tube 218 , which connects the chamber 213 with the regulatable blower 217 . by varying the heating output , the temperature of the air supplied to the chamber 213 can thus be varied , as a result of which the tongues 212 / 214 of the nozzles in the sheet guide baffle 213 b are deformed , and correspondingly decrease or increase the cross section of the nozzle openings . the heating output and the rpm of the blower 217 are adjusted by the control unit 219 in such a manner that the flow speed c of the airstream emerging from the nozzles and the volumetric air flow q can be adapted independently of one another to optimal conditions for the sheet 201 . controlled bending or warping of the resilient tongues of the nozzle openings can also be accomplished in other ways , however , such as are illustrated in fig8 a , wherein the tongues 312 of the nozzles in the guide baffle 313 b of the guide member have eccentric disks 316 assigned thereto , which rotate about a shaft 315 and with the aid of which the flexible tongues 312 can be partly closed . the shaft 315 is connected , for example , to a non - illustrated stepping motor , which in turn is also connected to a control unit by which the angular position of the shaft 315 , and thus the cross - sectional area of the nozzles , and optionally the power of the compressed air supplier can be adjusted . the cross - sectional area of the nozzle openings can naturally be varied by blocking members such as electrically actuatable slides 318 , flaps , or the like , as well , as shown in fig8 b , without requiring the nozzle tongues themselves to be resiliently constructed and deformed . in fig9 a particularly preferred embodiment of the invention is shown , wherein the guide member 413 , by which the sheet 401 is guided , is divided into three parallel chambers 416 a , 416 b and 416 c extending side by side in the travel direction of the sheet , each of the chambers 416 a to 416 c being connected to its own air supplier . the chambers 416 a and 416 c are each connected to a respective blower 417 a and 417 c , with which relatively high initial or supply pressures p 0 at low volumetric flows can be attained . the middle chamber 416 b is supplied by an axial fan 417 b , which already furnishes high volumetric air flows at even slight pressure differences . the boundaries of the three chambers need not extend in a straight line as shown in fig9 , but may instead have a zigzag course , so that different air flows emerging from the nozzles 412 a , b and c of the guide baffle 413 b , as described further hereinafter , can be mixed as well as possible under the guided sheet 401 . the chambers 416 a and 416 c , on the one hand , and the chamber 416 b , on the other hand , have markedly different consumer characteristic curves . correspondingly , volumetric flows of quite different magnitudes flow through the associated nozzles 412 a and 412 c , on the one hand , and 412 b , on the other hand , and these flows have flow speeds which differ sharply from one another . by mixing the supporting air flows , a mean value of the flow speed for the added volumetric flows is established , based upon the mixing rule . by purposeful changes in the parameters in the chambers 416 a and 416 c , on the one hand , and 416 b , on the other hand , it is now possible for the supporting air flow effectively acting upon the sheet and referred to the flow speed c , and the volumetric flow q , to be adjusted independently of one another . this is illustrated in further detail hereinbelow in conjunction with the graph in fig1 , wherein the consumer characteristic curve a 1 for the middle chamber 416 b is shown as having a relatively steep course . at low pressure , a high volumetric flow is already achieved . consequently , a large air volume with a low flow speed is blown under the sheet 401 . this is accomplished by a large number of nozzles in the sheet guide baffle 413 a or , in other words , a high nozzle density or by nozzles with openings having suitably large cross sections . the consumer characteristic curves a 2 of the two outer chambers 416 a and 416 c , conversely , have a relatively flat course . to force enough air through the nozzles , a high pressure difference is applied . because the pilot or supply pressure in the chambers 416 a and 416 c is consequently quite high , the air flows at high speed out of the nozzles associated with these chambers . this is accomplished by a low nozzle density or by providing nozzles with very narrow throttle cross sections . in the graph of fig1 , the characteristic curves of the chambers 416 a , b and c and the blower 417 are plotted jointly . for the two different types of chambers , in conjunction with the two different blowers or fan types , the operating points marked p 1 and p 2 then result . due to the arrangement of the nozzles 412 b , the air flowing out of the middle chamber 416 b is given a flow direction oriented towards the two outer chambers 416 a and c . the large air volume of the middle chamber 416 b flows at low speed between the guide baffle 413 a and the sheet 401 . this middle chamber 416 b is relatively narrow . above the two outer chambers 416 a and 416 c , the air emerging from the middle chamber 416 b mixes with that from the chambers 416 a and 416 c . the volumetric currents are added together there , and the speeds mix in a manner that is weighted in accordance with the proportions of the volumetric currents . if very different consumer characteristic curves for the chambers are selected , there results a broad spectrum of the operating points attainable by mixing the two airstreams . in this manner , simply by only a suitable control of the rpm of the gas blower 417 a and c or of the axial fan 417 b , the volumetric flow blown into the air cushion and the mean flow speed { overscore ( c )} established in accordance with the mixing rule can then be adjusted independently of one another , and thus the flotation height or level of the sheet 401 above the guide baffle 413 b can also be selected as required within broad limits . the extent to which the flotation height or level can be varied naturally depends upon the ratio of the cross - sectional areas of the nozzle openings of the chambers 416 a and c , on the one hand , and 416 b , on the other hand . a factor of approximately 2 to 20 is desired . the flotation height or level of the sheet established in accordance with or based upon this formula is shown in the three - dimensional graph of fig1 . the pressures in the chambers 416 a and c , on the one hand , and 416 b , on the other hand are plotted on the two abscissas , while the flotation level or height is plotted on the ordinate . it is assumed that the air generators and the air consumers have the characteristic curves shown in the graph of fig1 . it is apparent from the graph that the flotation height of the sheet can be changed somewhat by a factor of three , by varying the pressures in the chambers 416 a , b and c , the corresponding blowers 417 a , b and c being varied by the control unit 419 , for example , in accordance with the specification of the desired flotation height . the adaptation or matching can be performed in accordance with the following parameters : a ) the pressure in the middle chamber 416 b is changed . as a result , the volumetric flow of the entire air cushion guide changes very sharply , while the mean speed of the air flow remains virtually unchanged . the flotation height thereat changes approximately in proportion to the change in the volumetric flow originating in the middle chamber 416 b . b ) stability : observations indicate that the resistance offered by the air cushion guide to the sheet entering the air cushion and thus to an approach to the sheet guide baffle 413 b depends above all upon the flow speed . the higher the flow speed , the greater is the tendency of the airstream to stay where it is and thus the higher is the reaction force against disturbances of the air cushion . to increase the guidance stability , the pressure of the outer chambers 416 a and 416 b can therefore be increased . thin papers react to excessively high flow speeds by high - frequency vibrations . the pressure in the two outer chambers can be reduced thereat in order to accomplish a “ gentler ” guidance at approximately the same floating level . a further preferred exemplary embodiment of the invention is illustrated in fig1 a and b , wherein the sheet 501 to be printed , for example , on the rear side thereof in perfecter printing , is fed from a transfer drum 502 between two impression cylinders 503 and 504 . to assure reliable , smearfree sheet travel between the printing units , an air cushion guide of semicircular cross section is disposed below the transfer drum , spaced slightly apart from the arc described by the grippers of the drum 502 . the plan view on the likewise semicircular sheet guide baffle of this air cushion guide is shown in fig1 b . the broken lines therein indicate that the chamber 513 below the nozzle openings is subdivided into various regions 513 a to 513 h , and respective air generators 517 a to h are assigned to each partial chamber or subchamber . the air generators 513 a and b and 513 g and h are gas blowers , which supply a relatively high initial or supply pressure p to the few nozzles disposed on the outer edges of the sheet guide . conversely , the four middle regions disposed in succession in the sheet travel direction , i . e ., 513 c , d , e and f , and the nozzles associated therewith and disposed so close together on the sheet guide baffle that they engage one another , are supplied by four axial fans 517 c , d , e and f , which generate a high volumetric flow . the adjustment of the flotation height h of the sheet 501 above the sheet guide baffle is effected in a manner similar to that described for the exemplary embodiment of fig9 .