  We made CO(1-0) observations of 103 lines of sight in the core and envelope
of the high-latitude cloud MBM 40 to determine how the CO-H_2 conversion factor
(X_CO) varies throughout the cloud. Calibrating X_CO with CH data at similar
resolution (1' for CO, 1.5' for CH) yields values of X_CO ranging from 0.6
10^20 to 3.3 10^20 cm^-2 [K km s^-1]^-1 with an average of 1.3 10^20 cm^-2 [K
km s^-1]^-1. Given that the cloud has a peak reddening of 0.24 mag, it should
be classed as a diffuse rather than a translucent molecular cloud. The mass
obtained from the CO data and our values of X_CO is 9.6 M(solar) for the core,
12 M(solar) for the envelope, and 10 M(solar) for the periphery of the cloud. A
third of the molecular mass of the cloud is found in a region with E(B-V) <
0.12 mag. With these mass estimates, we determine that the cloud is not
gravitationally bound.
