The budget-making authority may, from time to time, direct the treasurer to invest, such portion of such fund as in its opinion is advisable, provided: (1) Not more than thirty-one per cent of the total amount invested shall be invested in equity securities; and (2) not less than fifty per cent of the total amount invested shall be invested in the stock or bonds or interest-bearing notes or obligations of the United States, or those for which the faith of the United States is pledged to provide for the payment of the principal and interest, including the bonds of the District of Columbia.