Interest upon interest, i. e., when the interest of a sum of money Is added to the principal, and then bears interest, which thus becomes a sort of secondary principal. Camp v. Bates, 11 Conn. 487; Woods v. Rankin, 2 Heisk. (Tenn.) 40; U. S. Mortg. Co. v. Sperry (C. C.) 26 Fed. 730