Upon the happening of any emergency caused by fire, flood, storm, epidemic, riot or insurrection, or caused by an inherent defect due to defective construction, or when the immediate preservation of order or of public health is necessary, or when the restoration of a condition of usefulness of any public building or other property which has been destroyed by accident or otherwise, is necessary, or when mandatory expenditures required by law must be met, the board of supervisors may, upon adoption, by unanimous vote of all members present at any meeting, of a resolution stating the facts constituting the emergency and entering the same on its minutes, make the expenditures, borrow money or incur the liabilities necessary to meet such emergency, without further notice or hearing, and may revise the budget accordingly. The clerk then shall be authorized to issue emergency warrants drawn against the special fund or funds properly chargeable with such expenditures, and upon presentation of such warrants the county depository shall pay same with any money in such fund or funds available for such purpose. If at any time there shall not be sufficient money available in such fund or funds, from usual sources or from grants, transfers or donations, to pay such warrants, the board of supervisors of the county is hereby authorized to borrow the required amount, not to exceed the authorized emergency expenditure, and shall execute the notes of the county for the amount so borrowed, and the board of supervisors, in such event shall be authorized to levy a special tax, not to exceed two mills on the dollar, for the repayment of such notes, which shall be made to mature not later than the 15th day of March next succeeding the date of issuance.