Bonds issued hereunder shall mature in such annual series or instalments, equal or unequal in amount, as the municipal council shall provide, except that:
(1) The first maturing bonds shall mature within five years from the date as of which they are issued;
(2) Not less than two per cent of the aggregate of the issue shall mature in any year; and
(3) No bonds shall mature later than forty years from the date as of which they are issued.