53 Pa. Stat. § 22675
It shall be the duty of said Sinking Fund Commission to establish and maintain a fund for the reduction and retirement of the funded indebtedness of said cities, the whole or any part of which is outstanding. Proper accounts shall be kept thereof; and there shall be appropriated to and set apart annually for the Sinking Fund Commission, for each issue of bonds, such nearly equal amounts at least, as appropriated annually over the term of such bonds, as will, at the earning or established rate of interest, at the maturity of each issue of bonds equal the total amount of such issue. The Sinking Fund Commission shall provide each year an amortization schedule, to be based upon the actual earning rate of interest on its investments; and this amortization schedule shall be a calculation taking into account any amounts of loss or gain there may be by reason of a lesser or greater rate of interest being received than that used in the calculation for the preceding annual schedule. Said commission shall always have on hand investments and moneys at least equal in amount to the total sum of such payments into the sinking fund, plus the accrued interest earnings calculated on said basis.
53 P.S. § 22675