referring to fig1 and 2 , a pneumatic apparatus with external drive ( 10 ) is shown comprising a pressure vessel ( 12 ) connected to the compressed gas outlet ( 16 ) and containing several components . one of the components is a pneumatic cylinder ( 32 ) containing a piston ( 34 ) which is connected to the piston shaft ( 28 ). the piston shaft ( 28 ) is connected to one end of the piston shaft chain ( 24 ). the other end of the piston shaft chain ( 24 ) is circumferentially attached to the wheel ( 22 ) and some additional portion of the piston shaft chain ( 24 ) is in contact with the edge of the wheel ( 22 ) such that movement of the piston shaft ( 28 ) away from the wheel ( 22 ) will cause the wheel ( 22 ) to rotate . the wheel ( 22 ) is concentrically attached to the output shaft ( 38 ). the y valve switch operators ( 23 a and 23 b ) are attached to the side of the wheel ( 22 ). one end of the spring chain ( 26 ) is circumferentially attached to the wheel ( 22 ). the other end of the spring chain ( 26 ) is attached to the spring ( 30 ) and some additional portion of the spring chain ( 26 ) is in contact with the edge of the wheel ( 22 ) such that movement of the part of the spring ( 28 ) connected to the spring chain ( 26 ) away from the wheel ( 22 ) will cause the wheel ( 22 ) to rotate . the y valve ( 18 ) is positioned adjacent to the wheel ( 22 ) such that the y valve switch operators ( 23 ) contact the y valve switch ( 20 ) when the wheel ( 22 ) rotates . the y valve is connected to the compressed gas inlet ( 14 ), the valve to cylinder conduit ( 33 ) and the valve to interior conduit ( 35 ). the pressure vessel ( 12 ) is filled with sufficient oil ( 17 ) such that substantially all of the components contained in the pressure vessel ( 12 ) are below the top of oil ( 19 ) and are submerged in the oil ( 17 ). the piston seal ( 36 ) and piston shaft seal ( 31 ) act to exclude the oil ( 17 ) from the pneumatic cylinder ( 32 ). the compressed gas outlet ( 16 ) and the compressed gas inlet ( 14 ) are each connected to a point on a compressed gas system . the compressed gas system has pressure differentials within it . the compressed gas outlet ( 16 ) is connected to a point on the compressed gas system where the pressure is lower than the pressure at the point where the compressed gas inlet ( 14 ) is connected to the compressed gas system . the compressed gas outlet ( 16 ) is open to the interior of the pressure vessel ( 12 ) and therefore the interior of the pressure vessel ( 12 ) is at substantially the same pressure as the point on the compressed gas system where the compressed gas outlet ( 16 ) is connected . referring to fig1 the piston ( 34 ) is at the top of its stroke and the wheel ( 22 ) is in a position whereby one of the y valve switch operators ( 2 a ) is in contact with the y valve switch ( 20 ). the y valve switch ( 20 ) is in the position whereby the y valve ( 18 ) is only open between the compressed gas inlet ( 14 ) and the valve to cylinder conduit ( 33 ), which permits a flow of compressed gas from the compressed gas inlet ( 14 ) to the valve to cylinder conduit ( 33 ) and thereby into the pneumatic cylinder ( 32 ). as the compressed gas flows into the pneumatic cylinder ( 32 ) the pressure on the compressed gas side of the piston ( 34 ) tends to exceed the pressure on the other side of the piston ( 34 ) generating a force against the piston ( 34 ). when that force exceeds the resistence of the spring ( 30 ), the spring begins to stretch and the piston ( 34 ) begins to move towards the bottom of its stroke . when the piston ( 34 ) reaches the bottom of its stroke , fig2 the wheel ( 22 ) has been rotated by the movement of the piston ( 34 ) through the connection with the piston chain ( 24 ) and piston shaft ( 28 ), such that the other y valve switch operator ( 23 b ) contacts the y valve switch ( 20 ) and moves the y valve switch ( 20 ) so that the y valve ( 18 ) is only open between the valve to interior conduit ( 35 ) and the valve to cylinder conduit ( 33 ), which permits a flow of compressed gas from the pneumatic cylinder ( 32 ), through the valve to cylinder conduit ( 33 ), through the valve to interior conduit ( 35 ) and into the interior of the pressure vessel , whereby the pressure within the pneumatic cylinder ( 32 ) drops to equalize with the pressure in the remainder of the interior of the pressure vessel ( 12 ). as the pressure in the pneumatic cylinder ( 32 ) drops , the force against the piston ( 34 ) decreases to a point where it is less than the resistence of the spring ( 30 ) imparted via the spring chain ( 26 ), wheel ( 22 ), piston chain ( 28 ) and piston shaft ( 28 ), and the piston ( 34 ) moves towards the top of its stroke . when the piston ( 34 ) reaches the top of its stroke , fig1 a y valve switch operator ( 23 a ) contacts and moves the y valve switch ( 20 ) to the position whereby the y valve ( 18 ) is only open between the compressed gas inlet ( 14 ) and the valve to cylinder conduit ( 33 ), and the piston ( 34 ) begins another stroke . in this way , each cycle of the piston ( 34 ), from top of stroke , fig1 to bottom of stroke , fig2 and back again , fig1 causes the wheel ( 22 ) to rotate first in one direction and then in the other direction . the wheel ( 22 ) is fixed to the output shaft ( 38 ). the output shaft ( 38 ) undergoes the same oscillating rotary motion as the wheel ( 22 ). the output shaft ( 38 ) projects through the wall of the pressure vessel ( 12 ), fig3 and is sealed with a suitable seal ( not shown ). the output shaft ( 38 ) can be used to drive a variety of mechanical devices . referring to fig4 and 5 , a pneumatic apparatus with internal pump ( 41 ) is shown . an embodiment of the invention with a configuration of spring ( 30 ), spring chain ( 26 ), wheel ( 22 ), piston shaft chain ( 24 ) and piston shaft ( 28 ), which is similar to the previously described pneumatic apparatus with external drive , is shown . however , such a configuration is not essential to the invention . the invention can function with a variety of configurations comprising a pressure vessel ; a pneumatic drive unit ; a pump ; and a means of connecting the pneumatic drive unit to the pump . the pressure vessel ( 12 ) is filled with sufficient oil ( 17 ) such that substantially all of the components contained in the pressure vessel ( 12 ) are below the top of oil ( 19 ) and are submerged in the oil ( 17 ). the piston seal ( 36 ) and piston shaft seal ( 31 ) act to exclude the oil ( 17 ) from the pneumatic cylinder ( 32 ). the embodiment of the invention shown in fig4 and 5 , operates to create a reciprocating movement of the piston ( 34 ) in the same manner as the previously described pneumatic apparatus with external drive , but it is different in that the wheel axis of rotation ( 43 ) is not necessarily an output shaft . the piston ( 34 ) is connected to the pump ( 42 ) by way of the pump drive shaft ( 48 ). the pump is connected to a liquid inlet ( 44 ) and a liquid outlet ( 46 ). the liquid inlet ( 44 ) and liquid outlet ( 46 ) pass through the wall of the pressure vessel ( 12 ). the liquid inlet ( 44 ) is connected to a supply of the liquid to be injected ( not shown ). the liquid outlet ( 46 ) is connected to whatever the liquid is being pumped to , for example a compressed gas system where the liquid is being injected ( not shown ). the reciprocating movement of the piston ( 34 ) drives the pump ( 42 ), which pumps the liquid from the liquid inlet ( 44 ) to the liquid outlet ( 46 ). referring to fig6 an embodiment of the liquid injection apparatus is shown . a gas processing pressure vessel ( 52 ) is connected to an upstream pipeline ( 68 ) and a downstream pipeline ( 70 ). the gas processing pressure vessel ( 52 ) can contain means for processing the gas within it ( not shown ), for example , it may be a dehydrator . the gas processing pressure vessel ( 52 ) contains the liquid container ( 54 ). the liquid container ( 54 ) contains a liquid ( 55 ). the liquid container ( 54 ) is connected to the liquid injection line ( 62 ). the liquid injection line passes through the wall of the gas processing pressure vessel and connects to the downstream pipeline ( 70 ). the metering valve ( 64 ) and the meter ( 66 ) are on the liquid injection line ( 62 ). the liquid fill line ( 56 ) passes through the wall of the gas processing pressure vessel and empties into the liquid container ( 54 ). the shut - off valve ( 58 ) is on the liquid fill line ( 56 ). the liquid container is connected by suitable conduit passing through the wall of the gas processing pressure vessel ( 52 ) to the level sight glass ( 62 ). the liquid container ( 54 ) is filled with the liquid ( 55 ) by means of the liquid fill line ( 56 ). when filling is not taking place the shut - off valve ( 58 ) is closed so as to maintain the pressure of the compressed gas in the gas processing pressure vessel ( 52 ). it is apparent that there are many other means of filling the liquid container , such as through the liquid injection line ( 62 ) with the addition of a t fitting and valve ( not shown ). the level sight glass ( 60 ) or other suitable device can be used to monitor the level of the liquid ( 55 ) in the liquid container ( 54 ). the interior of the liquid container ( 54 ) is exposed to the same pressure as the exterior of the liquid container ( 54 ) by suitable means . therefore , the liquid ( 55 ) in the liquid container ( 54 ) is under the same pressure as the compressed gas flowing in the pipeline ( 68 and 70 ) and filling the gas processing pressure vessel ( 54 ). the liquid . ( 55 ) in the liquid container ( 54 ) can flow by gravity into the liquid injection line ( 62 ) into the downstream pipeline ( 70 ). the metering valve ( 64 ) can be used to control the flow of liquid into the downstream pipeline ( 70 ). the meter ( 66 ) can be used to monitor the flow of liquid into the downstream pipeline ( 70 ). a further embodiment of the invention is shown in fig7 and 9 . the pneumatic apparatus ( 110 ) comprises a pressure vessel ( 112 ) containing a pneumatic device ( 113 ). a trunnion ( 114 ) is attached to , and passing through , the side of the pressure vessel ( 112 ). the components of the pneumatic device include a main bracket ( 116 ), a pneumatic cylinder ( 118 ), a y valve ( 120 ) and a drive sprocket ( 126 ). the pneumatic cylinder ( 118 ) and y valve ( 120 ) are mounted on the main bracket ( 116 ). the trunnion ( 114 ) passes through , and is attached to , the main bracket ( 116 ). the trunnion ( 114 ) has a trunnion bore ( 122 ) in which the output shaft ( 124 ) is disposed . the drive sprocket ( 126 ) is concentrically mounted on the output shaft ( 124 ). in the embodiment shown in fig7 and 9 , the pressure vessel ( 112 ) comprises the body ( 128 ) and the blind flange ( 130 ). the body ( 128 ) has a body flange ( 132 ) to which the blind flange ( 130 ) is bolted ( bolts not shown ) when the apparatus is in use . a gasket ( 134 ) is positioned between the blind flange ( 130 ) and the body flange ( 132 ). the blind flange ( 130 ) has a gas inlet ( 136 ) and a gas outlet ( 138 ). it will be clear to persons skilled in the art that the pressure vessel ( 112 ) can take many different forms . the gas inlet ( 136 ) is a port through the blind flange ( 130 ). on the top of the blind flange ( 130 ), the gas inlet ( 136 ) is , when in use , connected by suitable conduit , the inlet pipe ( 140 ), to the relatively higher pressure area in a compressed gas system . in the interior of the pressure vessel ( 112 ), the gas inlet ( 136 ) is connected by the inlet conduit ( 141 ) to the pneumatic device intake ( 143 ) on the y valve ( 120 ). the gas outlet ( 138 ) is a port through the blind flange ( 130 ). on the top of the blind flange ( 130 ), the gas outlet ( 138 ) is , when in use , connected by suitable conduit , the outlet pipe ( 142 ), to the relatively lower pressure area in a compressed gas system . in this way , the interior of the pressure vessel ( 112 ) is in fluid contact with the relatively lower pressure area in the compressed gas system and is therefore at substantially the same pressure . the inlet pipe ( 140 ) and the outlet pipe ( 142 ) are typically metal pipe . in fig7 and 9 , screwed pipe is shown . as shown in fig9 the trunnion ( 114 ) is screwed into a threaded trunnion port ( 144 ). the trunnion ( 114 ) contains two bushings ( 146 ) within which the output shaft ( 124 ) rotates . the bushings ( 146 ) must be made of a material which will not , under the normal working pressure in the interior of the pressure vessel ( 112 ), absorb gas or liquid so as to swell and bind between the trunnion ( 114 ) and the output shaft ( 124 ). bushings made of polyetheretherketone are suitable for working pressures of at least 1 , 000 pounds per square inch . output shaft seals ( 148 ) are disposed within the trunnion ( 114 ) and around the output shaft ( 124 ). the output shaft seals ( 148 ) stop gas from leaking from the pressure vessel ( 112 ) between the trunnion ( 114 ) and the output shaft ( 124 ), while permitting the output shaft ( 124 ) to rotate . commercially - available , urethane - based , hydraulic ram seals are suitable for the output shaft seals ( 148 ). the drive sprocket ( 126 ) has a concentric bore ( not shown ) sized to slide over the output shaft ( 124 ). the concentric bore of the drive sprocket ( 126 ) and the exterior of the output shaft ( 124 ) each have a key way ( not shown ). a key ( not shown ) is inserted into the key way to prevent the drive sprocket ( 126 ) and output shaft ( 124 ) from rotating relative to each other . a set screw ( not shown ) in the hub of the drive sprocket ( 126 ) is used to prevent the drive sprocket ( 126 ) from moving lengthwise on the output shaft ( 124 ). in use , the pressure in the interior of the pressure vessel ( 112 ) may be several hundred pounds per square inch greater than the exterior ambient air pressure . pressure differentials of this magnitude will result in a force of several hundred pounds tending to push the output shaft ( 124 ) out of the pressure vessel ( 112 ). to deal with this force , one end of the bore in the drive sprocket ( 126 ) has a circumferential counter bore ( not shown ), and the output shaft ( 124 ) has a circumferential groove ( not shown ). the counter bore and groove are sized and positioned to contain a split ring ( not shown ), wherein the split ring is positioned in the groove and the drive sprocket ( 126 ) is slid into position on the output shaft ( 124 ) and fixed in position with the set screw so as to retain the split ring . a thrust bearing ( 150 ) is disposed between the trunnion ( 114 ) and the drive sprocket ( 126 ). in this way , the force pushing the output shaft ( 124 ) out of the pressure vessel ( 112 ) bears against the split ring . in addition , a safety nut ( 152 ) is threaded on to the output shaft ( 124 ) in case the thrust bearing ( 150 ) and split ring fail . the pneumatic cylinder ( 118 ) contains a piston ( 160 ). the piston ( 160 ) has a piston seal ( 162 ) around its periphery which abuts the interior of the pneumatic cylinder ( 118 ) through the full range of movement of the piston ( 160 ). the piston seal ( 162 ) acts to seal the chamber ( 164 ) defined by the piston ( 160 ) and the pneumatic cylinder ( 118 ) from the rest of the interior of the pressure vessel ( 112 ). the piston ( 160 ) is in contact with the piston spring ( 166 ). the piston spring ( 166 ) is contained between the piston spring housing ( 168 ) and the piston ( 160 ). the piston spring ( 166 ) is under compression and the piston spring ( 166 ) causes the piston ( 160 ) to tend to move towards top of stroke , which position of the piston ( 160 ) is shown in fig7 . the piston ( 160 ) is attached to one end of the ram ( 170 ). the ram ( 170 ) passes through the ram seal ( 172 ) at the end of the pneumatic cylinder ( 118 ). the ram seal ( 172 ) acts to seal the chamber ( 164 ) from the rest of the interior of the pressure vessel ( 112 ). the other end of the ram ( 170 ) is connected to one end of the piston chain ( 180 ). the other end of the piston chain ( 180 ) is circumferentially attached to the drive sprocket ( 126 ). the drive sprocket ( 126 ) has teeth ( 182 ) spaced evenly around its periphery . the links of the piston chain ( 180 ) and the teeth ( 182 ) are sized to engage with each other . some portion of the links of the piston chain ( 180 ) proximate to the end of the piston chain ( 180 ) which is attached to the drive sprocket ( 126 ) are engaged with the teeth ( 182 ). movement of the ram ( 170 ) away from the drive sprocket ( 126 ) will cause the drive sprocket ( 126 ) to rotate in a clockwise direction , as the drive sprocket ( 126 ) is shown in fig7 and 8 . a second chain , the return chain ( 184 ), is circumferentially attached to the drive sprocket ( 126 ). the links of the return chain ( 184 ) and the teeth ( 182 ) are sized to engage with each other . some portion of the links of the return chain ( 184 ) proximate to the end of the return chain ( 184 ) which is attached to the drive sprocket ( 126 ) are engaged with the teeth ( 182 ). the end of the return chain ( 184 ) which is not attached to the drive sprocket ( 126 ) is attached to one end of the return spring ( 186 ). the other end of the return spring ( 186 ) is attached to the exterior of the pneumatic cylinder ( 118 ). the return spring ( 186 ) is under tension , such that the force exerted by the return spring ( 186 ) on the drive sprocket ( 126 ) through the return chain ( 184 ), causes the drive sprocket ( 126 ) to tend to rotate in a counterclockwise direction , as the drive sprocket ( 126 ) is shown in fig7 and 8 . the y valve ( 120 ) is connected to : the inlet conduit ( 141 ), the pneumatic cylinder conduit ( 190 ) and the exhaust area conduit ( 192 ). the inlet conduit ( 141 ) connects the pneumatic device intake ( 143 ) to the gas inlet ( 136 ). the inlet conduit ( 141 ) is typically metal tubing . the pneumatic cylinder conduit ( 190 ) connects the y valve ( 120 ) to the chamber ( 164 ). the pneumatic cylinder conduit ( 190 ) is typically metal tubing . the exhaust area conduit ( 192 ) connects the pneumatic device exhaust ( 193 ) on the y valve ( 120 ) to the exhaust area ( 194 ). in use , exhaust area ( 194 ) is an area which is in fluid communication with the relatively lower pressure portion of the compressed gas system . in fig7 and 9 , the exhaust area ( 194 ) is within the outlet pipe ( 142 ). this configuration prevents any liquids which might be mixed with the compressed gas from being deposited within the pressure vessel ( 112 ). if there is no concern about liquids or other matter being deposited in the pressure vessel ( 112 ), the exhaust area ( 194 ) can be within the pressure vessel ( 112 ). if the exhaust area is within the outlet pipe ( 142 ), the exhaust area conduit ( 192 ) is typically flexible tubing to facilitate fitting it within the outlet pipe ( 142 ). if the exhaust area ( 194 ) is within the pressure vessel ( 112 ), the exhaust area conduit ( 192 ) is typically rigid tubing . the y valve ( 120 ) is actuated by moving the valve switch ( 200 ) between the left stop ( 202 ) and the right stop ( 204 ). as with the directions of rotation of the drive sprocket ( 126 ), “ left ” and “ right ” are used solely for convenience in explaining the parts of the invention as shown in the views in fig7 and 8 . when the valve switch ( 200 ) is proximate to the left stop ( 202 ), gas can flow from the gas inlet ( 136 ), through the y valve ( 120 ) and into the chamber ( 164 ). when the valve switch ( 200 ) is proximate to the right stop ( 204 ), gas can flow from the chamber ( 164 ), through the y valve ( 120 ) and to the exhaust area ( 194 ). the valve switch ( 200 ) is controlled by the switch operator ( 206 ). the switch operator ( 206 ) is attached to the side of the drive sprocket ( 126 ) adjacent to the y valve ( 120 ). the switch operator ( 206 ) is connected to one end of the switch spring ( 208 ) at the operator pin ( 210 ). the valve switch ( 200 ) is connected to the other end of the switch spring ( 208 ) at the switch pin ( 212 ). the switch spring ( 208 ) is under tension . as is apparent in fig7 and 8 , the position which the operator pin ( 210 ) must be in for the switch operator ( 206 ) to move the valve switch ( 200 ), is defined by a line passing through the center of the switch pin ( 212 ) and the center of the switch pivot ( 214 ). for example , if , as shown in fig7 the operator pin ( 210 ) is to the left of the line defined by the switch pin ( 212 ) and the switch pivot ( 214 ), and the valve switch ( 200 ) is against the left stop ( 202 ), then the switch operator ( 206 ) will not exert a rightward pull on the valve switch ( 200 ) until the operator pin ( 210 ) is to the right of the line defined by the switch pin ( 212 ) and the switch pivot ( 214 ), at which time the operator pin ( 210 ) will be in position to pull the valve switch ( 200 ) against the right stop ( 204 ). it is clear that the same applies to the reverse motion of the switch operator ( 206 ) and valve switch ( 200 ). in this way , a reciprocal motion of the drive sprocket ( 126 ) causes the y valve ( 120 ) to be positively switched from one position to another . typically , the pressure vessel ( 112 ) is filled with sufficient oil ( 220 ) such that substantially all of the components contained in the pressure vessel ( 112 ) are below the top of oil ( 222 ) and are submerged in the oil ( 220 ). a dipstick assembly ( not shown ), comprising a threaded port in the top of the pressure vessel ( 112 ); a threaded plug rated for the working pressure of the apparatus ; and a marked rod attached to the threaded plug , may be installed to monitor the level of the oil ( 220 ) within the pressure vessel ( 112 ). in use , as described above , the gas inlet ( 136 ) is connected by suitable conduit to a relatively higher pressure area in a compressed gas system , and the gas outlet ( 138 ) is connected by suitable conduit to a relatively lower pressure area in the compressed gas system . the pressure differential between the higher pressure and lower pressure gas , drives the piston ( 160 ) in the power stroke , being from top of stroke to bottom of stroke , and the piston spring ( 166 ) and the return spring ( 186 ) drive the piston ( 160 ) in the exhaust stroke , being from bottom of stroke to top of stroke . [ 0099 ] fig7 shows the position of the various components just after the end of the exhaust stroke : the piston ( 160 ) is at top of stroke ; the drive sprocket ( 126 ) has been rotated counterclockwise by the force exerted by the return spring ( 186 ); and the switch operator ( 206 ) has pulled the valve switch ( 200 ) against the left stop ( 202 ), thereby permitting gas to flow from the gas inlet ( 136 ) to the chamber ( 164 ). as the relatively higher pressure compressed gas flows into the chamber ( 164 ), the pressure within the chamber ( 164 ) increases until it is greater than the pressure on the other side of the piston ( 160 ), which results in a force against the piston ( 160 ). movement of the piston ( 160 ) is resisted by a variety of factors , including : the resistence of the piston spring ( 166 ) and the return spring ( 186 ); friction ; and the resistence of the device which the pneumatic apparatus ( 110 ) is driving . when the force against the piston ( 160 ) exceeds this resistence the piston ( 160 ) begins to move towards bottom of stroke , compressing the piston spring ( 166 ) and stretching the chain spring ( 186 ). when the piston ( 160 ) reaches bottom of stroke , fig8 the piston chain ( 180 ) has rotated the drive sprocket ( 126 ) such that the operator pin ( 210 ) has moved sufficiently so that the valve switch ( 200 ) has been pulled against the right stop ( 204 ). in this position , the y valve ( 120 ) permits gas to flow from the chamber ( 164 ) to the exhaust area ( 194 ). the pressure within the chamber ( 164 ) equalizes with the pressure in the exhaust area ( 194 ), which is substantially the same as the pressure within the pressure vessel ( 112 ) and in the lower pressure portion of the gas system . as the pressure in the chamber ( 164 ) drops , the force against the piston ( 160 ), resulting from the difference in pressure on either side of the piston ( 160 ) decreases to a point where it is less than the force applied by the piston spring ( 166 ) and the return spring ( 186 ), and the piston ( 160 ) begins to move toward top of stroke . as the piston ( 160 ) reaches top of stroke , the switch operator ( 206 ) will pull the valve switch ( 200 ) to the left stop ( 202 ), the components of the apparatus will once again be in the position shown in fig7 and the cycle will repeat itself . in this way , each cycle of the piston ( 160 ), from top of stroke , fig7 to bottom of stroke , fig8 and back again , fig7 causes the drive sprocket ( 126 ) and the output shaft ( 124 ) to rotate first in one direction and then in the other direction . as will be obvious to those skilled in the art , the output shaft ( 124 ) can be used to drive a variety of mechanical devices in a variety of different ways , including , but not limited to : attaching a crank ( 224 ) to the shaft as shown in fig7 and 9 ; and by converting the oscillating rotary motion of the output shaft ( 124 ) to non - oscillating rotary motion by means of a ratchet slip clutch and flywheel ( not shown ). the foregoing is a description of a preferred embodiment of the invention which is given here by way of example . the invention is not to be taken as limited to any of the specific features as described , but comprehends all such variations thereof as come within the scope of the appended claims .