The public authorities of every city of the second class, charged with the disbursements, expenditures, and appropriations, shall annually set aside, apportion, and appropriate, out of all taxes and income of the said cities, unto the board of pensions, a sum sufficient to maintain the pensions or compensations due under this act. If the total disbursements from the pension fund for the payment of pensions, refund of contributions of employés, and administration expenses shall exceed in any year the total revenues of the pension fund from the contributions of employés and the interest and premiums earned on investments and bank deposits, then the amount to be set aside, apportioned, and appropriated by the public authorities of such city of the second class to the pension fund shall not be less than the difference between the total disbursements and receipts of the pension fund, as aforesaid.