And be it enacted, That no Stipend which shall be augmented or modified by a Decree after the passing of this Act, shall be again augmented or modified until the Expiration of Twenty Years from and after the, Date of such Decree or Modification thereof; nor shall any such Stipend be augmented or modified at any future Period until the Expiration of Twenty Years from and after the Date of the last Decree of Modification thereof reflectively.