FUNCTION    VISLI (Z,X,Y,I)

C     [LINEAR INTERPOLATION]
C     PERFORM THE BACKWARD LINEAR INTERPOLATIONS REQUIRED BY THE HORIZON
C     ROUTINES. VISLI RECEIVES SUCH INTENSE USAGE THAT IT DELIBERATELY
C     ESCHEWS A CHECK FOR A ZERO DENOMINATOR, WHICH STILL OCCASIONALLY
C     CREATES OVERFLOWS.
C     Z     POINT AT WHICH INTERPOLATION IS MADE
C     X     ARRAY IN WHICH Z IS INTERPOLATED
C     Y     ARRAY FROM WHICH TO TAKE THE INTERPOLATED VALUE
C     I     AN INDEX FOR WHICH X(I-1).LE.Z.LE.X(I)
C     [05-MAY-74]

      VIRTUAL   X(1),Y(1)

      X1=X(I-1)
      X2=X(I)
      Y1=Y(I-1)
      Y2=Y(I)
      VISLI=Y1+(Y2-Y1)*((Z-X1)/(X2-X1))
      RETURN

      END