Described in detail in the main body of the paper is a circular slide rule designed to use for many calculations where the seismic velocity varies linearly with distance from a datum plane (usually horizontal), i.e., where V = V 0 + az. Here V 0 is the velocity at datum and a is the rate of change of velocity with respect to depth z. Table 1 lists, in the first column, data that must be given and, in the second column, what may be found from it by using the present slide rule. Except for that of line 10, all calculations are limited only by the accuracy of reading the scales, which is sufficient for most practical purposes. The calculation of line 10 is approximate only, but needs correcting over only about 5 percent of its usually used range; this correction is very simply made.