The legislative body shall invest only in notes, bonds, bills, certificates of indebtedness, warrants, or registered warrants which are legal investments for savings banks in the State, provided, that the board of supervisors of a county may, by a four-fifths vote thereof, invest in notes, warrants or other evidences of indebtedness of public districts wholly or partly within the county, whether or not such notes, warrants, or other evidences of indebtedness are legal investments for savings banks.