In addition to all other taxes of every kind now imposed by law or otherwise imposed by this title, there is hereby levied a license or privilege tax upon each person, firm, corporation, agent or officer engaged in the business of manufacturing and selling hydroelectric power in the State of Alabama for the privilege of engaging in such business; said license or privilege tax shall be due and payable in advance, on or before September 25 of each year, and shall be in sum equal to two fifths of a mill upon each kilowatt hour of hydroelectric power manufactured and sold during the preceding calendar year.