Two principal methods of theater ventilation may be arranged: in one the fresh air enters at or near the floor and rises upward to the ceiling, to be removed by suitable outlet flues; in this method the incoming air follows the naturally existing air currents; in the other method pure air enters at the top through perforated cornices or holes in the ceiling, and gradually descends, to be removed by outlets located at or near the floor line. The two systems are known as the "upward" and the "downward" systems; each of them has been successfully tried, each offers some advantages, and each has its advocates. In both systems separate means for supplying fresh air to the boxes, balconies, and galleries are required. Owing to the different opinions held by architects and engineers, the two systems have often been made the subject of inquiry by scientific and government commissions in France, England, Germany, and the United States. A French scientist, Darcet, was the first to suggest a scientific system of theater ventilation. He made use of the heat from the central chandelier for removing the foul air, and admitted the air through numerous openings in the floor and through inlets in the front of the boxes. Dr. Reid, an English specialist in ventilation, is generally regarded as the originator of the upward method in ventilation. He applied the same with some success to the ventilation of the Houses of Parliament in London. Here fresh air is drawn in from high towers, and is conducted to the basement, where it is sprayed and moistened. A part of the air is warmed by hot-water coils in a sub-basement, while part remains cold. The warm and the cold air are mixed in special mixing chambers. From here the tempered air goes to a chamber located directly under the floor of the auditorium, and passes into the hall at the floor level through numerous small holes in the floor. The air enters with low velocity, and to prevent unpleasant draughts the floor is covered in one hall with hair carpet and in the other with coarse hemp matting, both of which are cleaned every day. The removal of the foul air takes place at the ceiling, and is assisted by the heat from the gas flames. The French engineer Péclet, an authority on heating and ventilation, suggested a similar system of upward ventilation, but instead of allowing the foul air to pass out through the roof, he conducted it downward into an underground channel which had exhaust draught. Trélat, another French engineer, followed practically the same method.