When the employment of as a medium for first-difference interpolation was suggested in J.I.A. LXXII, 453, no mention was made of a corresponding medium for the amount of an annuity. It seemed that the matter might bear further investigation from this angle, and that it would be well to bring the allied continuous functions within the scope of the inquiry. These latter are simpler to deal with because they may be expressed in terms of only one variable (ηδ).