Regarding the amount of fresh outdoor air to be supplied to keep the inside atmosphere at anything like standard purity, authorities differ somewhat. The theoretical amount, 3,000 cubic feet per person per hour (50 cubic feet per minute), is made a requirement in the Boston theater law. In Austria, the law calls for 1,050 cubic feet. The regulations of the Prussian Minister of Public Works call for 700 cubic feet, Professor von Pettenkofer suggests an air supply per person of from 1,410 to 1,675 cubic feet per hour (23 to 28 cubic feet per minute), General Morin calls for 1,200 to 1,500 cubic feet, and Dr. Billings, an American authority, requires 30 cubic feet per minute, or 1,800 cubic feet per hour. In the Vienna Opera House, which is described as one of the best-ventilated theaters in the world, the air supply is 15 cubic feet per person per minute. The Madison Square Theater, in New York, is stated to have an air supply of 25 cubic feet per person. In a moderately large theater, seating twelve hundred persons, the total hourly quantity of air to be supplied would, accordingly, amount to from 1,440,000 to 2,160,000 cubic feet. It is not an easy matter to arrange the fresh-air conduits of a size sufficient to furnish this volume of air; it is obviously costly to warm such a large quantity of air, and it is a still more difficult problem to introduce it without creating objectionable currents of air; and, finally, inasmuch as this air can not enter the auditorium unless a like amount of vitiated air is removed, the problem includes providing artificial means for the removal of large air volumes. Where gas illumination is used, each gas flame requires an additional air supply--from 140 to 280 cubic feet, according to General Morin. A slight consideration of the volumes of air which must be moved and removed in a theater to secure a complete change of air three or four times an hour, demonstrates the impossibility of securing satisfactory results by the so-called natural method of ventilation--i. e., the removal of air by means of flues with currents due either to the aspirating force of the wind or due to artificially increased temperature in the flues. It becomes necessary to adopt mechanical means of ventilation by using either exhaust fans or pressure blowers or both, these being driven either by steam engines or by electric motors. In the older theaters, which were lighted by gas, the heat of the flames could be utilized to a certain extent in creating ascending currents in outlet shafts, and this accomplished some air renewal. But nowadays the central chandelier is almost entirely dispensed with; glowing carbon lamps, fed by electric currents, replace the gas flames; hence mechanical ventilation seems all the more indicated.