The county board of any county in this state is authorized to provide for the purchase and taking up of registered warrants, as provided for in section 77-2335, out of the sinking funds in the hands of the county treasurer whenever in the judgment of such county board the same shall be safe and expedient. Before so investing any sinking funds, the county board shall fix and prescribe and enter of record general directions and authority to such county treasurer as to the funds to be invested, specifying the funds to be so invested, the kind and amount of warrants to be so invested in, and in so doing shall, as far as the same may be practicable, continue to invest the sinking fund which shall last become due and payable. Not more than fifty percent of the money so collected on any given sinking fund shall be so invested in warrants at any given time. When practicable, the warrants drawn by any given authority shall be provided for from the sinking funds belonging to the organization issuing such warrant, and of such provisions the county board shall give the treasurer notice.