The Commissioners may borrow Money by Mortgage of the special and general Sewer Rates for making new Sewers, or inclosing open Sewers, and also for any private Improvement Expences, by Mortgage of the Rates respectively applicable to defray such Expences, and for that Purpose the Clauses of the " Commissioners Clauses Act, 1847," with respect to the Mortgages to be executed by the Commissioners, shall be incorporated with this Act; and in order to discharge the Principal Money borrowed as aforesaid on Security of any such Rates, the Commissioners shall in every Year pay off not less than One Thirtieth Part of any Principal Sum so borrowed.