1. AN APPARATUS FOR MEASURING GAS QUANTITIES COMPRISING A GAS TANK HAVING AN UPPER LIMITING WALL AND A GAS OUTLET NEAR THE TOP OF THE TANK, AND A LIQUID TANK AT A LEVEL HIGHER THAN THAT OF SAID GAS TANK, SAID LIQUID TANK HAVING A LOWER LIMITING WALL AND COMMUNICATING AT ITS TOP WITH A SOURCE OF CONSTANT GAS PRESSURE, EACH OF SAID TANKS COMPRISING A LIQUID EXCHANGING PORTION EXTENDING FROM THE BOTTOM THEREOF UPWARDS AND ADAPTED TO HOLD A LIQUID DURING AT LEAST ONE STAGE OF THE MEASURING OPERATION, A CONDUIT CONNECTING THE LOWER PARTS OF SAID GAS AND LIQUID TANKS AND PERMITTING LIQUID TO PASS BY GRAVITY FROM SAID LIQUID EXCHANGING PORTION OF THE LIQUID TANK TO SAID LIQUID EXCHANGING PORTION OF THE LIQUID TANK TO SAID LIQUID EXCHANGING PORTION OF THE GAS TANK THEREBY DISPLACING GAS FROM THE GAS TANK THROUGH SAID GAS OUTLET, THE HORIZONTAL CROSS-SECTION AREA OF SAID LIQUID