As long as any such Burden shall remain unpaid the Casualties of Superiority which shall become payable under any Contracts, Dispositions, or Charters of Feu, or "Writs by Progress for entering Heirs or Successors to be granted as aforesaid, as well as any Payments which may be received from the Grantees thereof in respect of the Construction of Roads, Sewers, or Drains, shall be invested, at the Sight of the Heritors and Presbytery, on such Securities and in such Manner as the Court of Teinds shall approve, as a Sinking Fund to meet the said Burden, and the Interest of the said Fund shall be paid to the Minister for the Time being; and as soon as the said Fund shall amount to a Sum sufficient to pay the said Burden, the same shall be paid off; and thereupon the Casualties of Superiority thereafter to become due shall form Part of the Income of the Minister for the Time being, and be payable to him.