The fourth catalogue of radio sources observed at 408 MHz with the Molonglo radiotelescope is presented. A total of 1349 sources is listed to a limiting flux density of 0�2 Jy, the catalogue being largely complete above 0� 25 Jy. The area of sky surveyed includes the Magellanic Clouds as well as a comparison region of similar angular extent. Detailed notes are provided on some of the Cloud sources, and maps are given for complex sources in the region of the Clouds. Those sources for which high-frequency or optical data are available are classified into two lists, one of Cloud members and the other of 'unidentified nonthermal sources', presumably not Cloud members.

For the case where the rate of exchange is measured by the growth of activity, y, in B y{t) = y (oo) [1 -exp (-*)](4) d In s/d In (y/y(°>)) = (exp(s) -l)/s = F(s) (5) The figure contains a plot of this function, too. In this case, of course, the fractional error in y cannot possibly be constant as y approaches zero; an estimate of -y as a function of y is needed for selecting an optimum reaction time or calculating the error of a particular determination.Contribution No. 1233 from the Gates and