def viss(t, d):
    """Viscosity of vapour as a function of temperature (deg C) and
    density (kg / m3)."""
    V1 = 0.407 * t + 80.4
    if t <= 350.0: return 1.0e-7 * (V1 - d * (1858.0 - 5.9 * t) * 1.0e-3)
    else: return 1.0e-7 * (V1 + d * (0.353 + d * (676.5e-6 + d * 102.1e-9)))