Sec. 8. INVESTMENT OF SURPLUS FUNDS. Whenever, in the opinion of the said pension board, there is on hand in said pension fund, a surplus over and above a reasonably safe amount to take care of current demands upon such funds, such surplus, or so much thereof as in the judgment of the said pension board is deemed proper, may be invested in securities of the United States, the State of Texas, or of counties, school districts, or municipal corporations. No investment shall be made, however, which does not meet with the approval of the city controller, if any, of such city.