In his lecture ‘Computing methods’ (J.S.S. Vol. VIII, pp. 76-7), D. H. Sadler describes a method of inverse interpolation invented by Dr Comrie. This note shows how the method may be used to solve for i the equation = K. The solution of this equation to a sufficient degree of accuracy is a minor but troublesome problem of office practice, and the writer hopes that the method here described will be of practical value. In addition to its simplicity and directness it has the considerable merit that it makes full use of what a calculating machine can do, and, in consequence, requires from the computer very little calculation. It will be seen that, apart from the making of a short difference table, the whole of the work is done on the machine.