Quantitative study of the westward drift (w.d.) was initiated by Bullard et al. (1950) and then followed by many others, using more or less similar methods (e.g. Yukutake, 1962; Harwood and Malin, 1976). More recently, MAGSAT results have enhanced the interest in the subject (see for example fig. 1 of Peddie, 1982). In the present paper, we compute the w.d. rate in the 1950--1980 time interval, using different methods, and we establish that the 1969--1970 secular acceleration impulse (the so-called ''jerk'') clearly shows up in the results.