  If the cluster formation rate is constant and the disruption time of clusters
depends on their initial mass as t_dis= t_4 x (M_i/10^4 Msun)^gamma, where M_i
is the initial mass of a cluster, t_4 is the disruption time of a cluster of
M_i=10^4 Msun and gamma is an exponent, then the values of t_4 and gamma can be
derived in a very simple way from the age and mass histograms of large
homogeneous samples of clusters with reliable age and/or mass determinations.
We demonstrate the method and derive the values of t_4 and gamma from cluster
samples in selected fields of four galaxies: M51, M33, SMC and the solar
neighbourhood. The values of the exponent gamma are the same within their
uncertainties in the four galaxies with gamma = 0.62 +- 0.06. However, the
disruption time t_4 of clusters of 10^4 Msun is very different in the different
locations. It is shortest in the inner region of M51 (t_4 = 40 Myr) and highest
in the SMC (t_4=8 Gyr).
