A regression line has been fitted to times of diffracted S observed at Canberra from 11 earthquakes. When corrections are made to the observed times to compensate for probable mislocations of the earthquakes, a value of 8.9 sec/deg is obtained for the travel time gradient, with a 95 per cent confidence interval estimate of 0.2 sec/deg. Assuming a core radius of 3473 km, this corresponds to an S velocity at the core-mantle boundary of 6.8 km/sec. This is 0.5 km/sec lower than the value given by Jeffreys (1939). The analysis also provides an estimate of the S time at 100° distance which is about 12 sec greater than that given in the Jeffreys-Bullen tables.
The results suggest that current models of the lowest part of the mantle are significantly in error. Revision of the model in this region should enable a more satisfactory reconciliation with free oscillation data.