The board may at any time, after considering the probable current demands upon the fund, determine what portion of the fund may be safely withdrawn from the current cash account for investment for revenue purposes.  All proceedings relating thereto shall be entered upon its records.  Investments shall be only in interest-bearing bonds of the United States, the state or any political subdivision thereof.  All income from such investments shall be a part of the fund.  All securities shall be deposited with the fiscal agent of the municipality, and its receipt therefor filed with the governing authority of the municipality.