Within thirty days after the first Monday in April in each year, the trustees of a township constituted out of the division of a township, shall meet together and jointly assess and levy on the taxable property, real and personal, within the limits of the original township, the taxes necessary to pay the interest on bonds issued under section 505.18 of the Revised Code, and to pay such sum as seems expedient to them for the redemption of the principal named in such bonds.