Sec. 3. INVENTORY AND APPRAISAL OF EQUIPMENT; DISPOSAL AND PURCHASE. The county engineer shall at the end of every 12 months, acting in conjunction with each commissioner of the county, make a complete inventory and appraisement of all tools, machinery, equipment, materials, trucks, cars, and other property owned by the respective commissioners, and transmit the same in written form to the commissioners court and the county auditor, which report shall be kept as a permanent inventory record by the county auditor. When any of said tools, machinery, trucks, cars, and other property becomes unusable, the commissioners court shall enter an order upon the minutes of the court, stating such facts and reason for disposing of such equipment and shall have authority to dispose of same as it deems best. When in its opinion it is necessary to purchase other machinery, supplies, tools, and other equipment and materials, the commissioners court shall enter an order on the minutes showing the necessity therefor. All equipment purchased or acquired as herein specified, shall be shown on the permanent inventory record.