1. A HYDRAULIC COMPUTING SYSTEM FOR OBTAINING THE PRODUCT OF SENSED VARIABLES, MEANS FOR PRODUCING A PRESSURE DROP WHOSE VALUE IS INDICATIVE OF ONE OF THE SENSED VARIABLES TO BE MULTIPLIED, MEANS INCLUDING A PAIR OF FLOW LINES EACH OF WHICH CONTAIN A VARIABLE RESTRICTION, ONE OF WHICH VARIES AS A FUNCTION OF ANOTHER SENSED VARIABLE FOR MODIFYING SAID PRESSURE DROP TO PRODUCE ANOTHER PRESSURE DROP WHOSE VALUE IS INDICATIVE OF THE PRODUCT OF THE TWO VARIABLES, AND MEANS FOR INTERCONNECTING SAID VARIABLE RESTRICTIONS SO THAT WHEN ONE MOVES TO AN OPENED POSITION THE OTHER MOVES TO A CLOSED POSITION.