For the Purpose of providing a Fund for paying off Mortgages granted under this Act, the Board or Vestry granting such- Mortgage shall once in every Year set aside, out of the Monies or Rates charged thereby, such Sum as they think proper, being not less than Two Pounds per Centum on the Amount of the Principal Monies secured thereby; and the Sum so from Time to Time set aside, and all other Monies applied by the Board or Vestry in augmentation of the said Fund, shall be applied, in the Manner herein-after directed, in Payment, so far as the same will extend, of the Principal Money secured by such Mortgages, or the same shall be invested in the Public Funds, or on Government or Peal Security, in the Name of the Board or Vestry; and the Dividends and Interest of the Monies so invested, when and as the same become due, shall from Time to Time be received and invested in like Manner, in order that the said Monies so set aside and invested may accumulate at Compound Interest; and when such accumulated Fund amounts to a Sum which, in the Opinion of the Board or Vestry, can be conveniently applied for that Purpose, the Stocks, Funds, or Securities whereon the same is invested shall be sold or otherwise converted into Money, and the Monies arising from any such Sale and Conversion shall be applied, in the Manner herein-after directed, in Payment, so far as the same will extend, of the said Principal Monies, and so from Time to Time until the whole of the said Principal Monies, are discharged.