The speed-cones are either continuous cones or **conoids**, as A, B, whose velocity ratio can be varied gradually while they are in motion by shifting the belt, or sets of pulleys whose radii vary by steps, as C, D, in which case the velocity ratio can be changed by shifting the belt from one pair of pulleys to another.

The following is the most convenient practical rule for the application of this equation: Let the speed-cones be equal and similar **conoids**, as in B, fig.

Let r1 be the radius of the large end of each, ri that of the small end, r, that of the middle; and let Ii be the sagitta, measured perpendicular to the axes, of the arc by whose revolution each of the **conoids** is generated, or, in other words, the bulging of the **conoids** in the middle of their length.

In his extant **Conoids** and Spheroids he defines a conoid to be the solid formed by the revolution of the parabola and hyperbola about its axis, and a spheroid to be formed similarly from the ellipse; these solids he discussed with great acumen, and effected their cubature by his famous "method of exhaustions."