​ARITHMETICAL PROGRESSION, a series of numbers increasing or diminishing uniformly by the same number. If they increase, the arithmetical progression is said to be ascending, and if they decrease, descending. Thus the series 3, 6, 9, 12, 15 is an ascending arithmetical progression, mounting up by the continued addition of 3; and the series 8, 6, 4, 2 is a descending one, falling regularly by 2.