(70 ILCS 2105/15) (from Ch. 42, par. 398)

Sec. 15. At the time of or before incurring any indebtedness, the board of trustees shall provide for the collection of a direct annual tax sufficient to pay the interest on such debt as it falls due, and also to pay and discharge the principal thereof as the same shall fall due, and at least within twenty years from the time of contracting same.

(Source: Laws 1925, p. 346.)