FUNCTION PCH4(T)
C THIS SUBROUTINE RETURNS THE VAPOR PRESSURE OF METHANE
C OVER THE TEMPERATURE INTERVAL (74-97K) IN UNITS OF BARS
C IT IS BASED ON DATA FROM THE MATHASEN GAS DATA BOOK P 351
C FITTED BY CP MCKAY 10/85
      PCH4=3.4543E4 * EXP(-1145.705/T)
      RETURN
      END