Arithmetical Progression

Arithmetical Progression is a series of numbers that increase or diminish by a common difference, as 7, 10, 13, 16, 19, 22; or, 12, 10½, 9, 7½, 6. To find the sum of such a series, multiply the sum of the first and last terms by half the number of terms. The series of natural numbers, 1, 2, 3, 4, &c. form an arithmetical progression, of which the difference is 1.