A large number of theaters are ventilated on the upward system. I will mention first the large Vienna Opera House, the ventilation of which was planned by Dr. Boehm. The auditorium holds about three thousand persons, and a fresh-air supply of about fifteen cubic feet per minute, or from nine hundred to one thousand cubic feet per hour, per person is provided. The fresh air is taken in from the gardens surrounding the theater and is conducted into the cellar, where it passes through a water spray, which removes the dust and cools the air in summer. A suction fan ten feet in diameter is provided, which blows the air through a conduit forty-five square feet in area into a series of three chambers located vertically over each other under the auditorium. The lowest of these chambers is the cold-air chamber; the middle one is the heating chamber and contains steam-heating stacks; the highest chamber is the mixing chamber. The air goes partly to the heating and partly to the mixing chamber; from this it enters the auditorium at the rate of one foot per second velocity through openings in the risers of the seats in the parquet, and also through vertical wall channels to the boxes and upper galleries. The total area of the fresh-air openings is 750 square feet. The foul air ascends, assisted by the heat of the central chandelier, and is collected into a large exhaust tube. The foul air from the gallery passes out through separate channels. In the roof over the auditorium there is a fan which expels the entire foul air. Telegraphic thermometers are placed in all parts of the house and communicate with the inspection room, where the engineer in charge of the ventilation controls and regulates the temperature. The Vienna Hofburg Theater was ventilated on the same system. The new Frankfort Opera House has a ventilation system modeled upon that of the Vienna Opera House, but with improvements in some details. The house has a capacity of two thousand people, and for each person fourteen hundred cubic feet of fresh air per hour are supplied. A fan about ten feet in diameter and making ninety to one hundred revolutions per minute brings in the fresh air from outdoors and drives it into chambers under the auditorium arranged very much like those at Vienna. The total quantity of fresh air supplied per hour is 2,800,000 cubic feet. The air enters the auditorium through gratings fixed above the floor level in the risers. The foul air is removed by outlets in the ceilings, which unite into a large vertical shaft below the cupola. An exhaust fan of ten feet diameter is placed in the cupola shaft, and is used for summer ventilation only. Every single box and stall is ventilated separately. The cost of the entire system was about one hundred and twenty-five thousand dollars; it requires a staff of two engineers, six assistant engineers, and a number of stokers.