Sec. 9. INVESTMENT OF SURPLUS. Whenever in the opinion of the said Pension Board there is on hand in said Pension System 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 bonds or other interest bearing obligations and securities of the United States, the State of Texas or any city or any county.