The principal and interest of all revenue bonds shall be payable in lawful money of the United States of America at the office of the treasurer of the district issuing the same. Revenue bonds shall be serial in character and an approximately even proportion of the total amount of each issue shall be payable annually. The amount of the principal due in each annual payment excepting the last may be made to differ not more than five hundred dollars ($500) from the amount obtained by dividing the total of the principal amount due under the bonds, by the number of installments which shall not exceed four. The last installment shall be for the balance of the total principal amount

not provided to be paid in the previous installments.