They cannot change the average value, but they can determine the deviations from the average.

If the dominant banks manage ill, the rate of interest will at one time be excessively high, and at another time excessively low: there will be first a pernicious excitement, and next a fatal collapse.

But if they manage well, the rate of interest will not deviate so much from the average rate; it will neither ascend so high nor descend so low.
As far as anything can be steady the value of money will then be steady, and probably in consequence trade will be steady too--at least a principal cause of periodical disturbance will have been withdrawn from it..