Whenever the revenues of the city for any fiscal year should, before the end of such fiscal year, fall below the allowances set apart in the budget for the maintenance and operation of the various departments of the city government for that year, due to unavoidable shortage in the collection of taxes or other revenues relied upon by the city at the time of making the annual budget appropriation for said year, then, in such contingency, the council shall have the right to borrow, at the lowest obtainable rates, the amount necessarily required to conduct the operations of the city government for the remainder of the fiscal year; but the total amount of such indebtedness hereby authorized to be created shall not exceed ten (10) per cent of the total budget appropriation. The council shall, as security for the repayment of said loan, be authorized to pledge the uncollected taxes for said fiscal year, and it shall be their duty to retire the said loan out of the proceeds of said taxes as fast as collections thereof shall be made.