The plan of ridge ventilation, shown in the accompanying section, I applied first in St. Louis, in the summer of 1863. It is the principle of the Emerson ventilator applied to ridge ventilation. Much trouble had been experienced with other forms on account of their allowing the storms to beat in, and the difficulty of opening and closing them with the various changes of wind; this form fully remedies those objections, and can be left open without inconvenience at all times while snowing or raining. It uses the force of the wind, whenever there is a current passing over the top of the building, for sucking the air out of the ward, because the air in passing across the top of the building is deflected from the straight line by the angle of the roof-board, which creates a partial vacuum in the space below, which, with the friction of the passing current with that coming out of the ward, makes an outward draught, varying in proportion to the velocity of the external current. This is often very useful, especially in summer, when there is not sufficient difference between the external air and that in the ward to create a current. There is often a considerable force in the passing current at the top of the building when there is much less below. [Illustration: Fig. 2.] But of course these openings had to be closed in winter to prevent all the heat from escaping. It then became necessary in wards that had no fireplaces, to make something as substitutes therefor. Wooden shafts or flues were made to answer this purpose. I at first made large wooden boxes, placing them in the centre of the wards, and allowing them to extend down to within twelve or eighteen inches of the floor. This was of great advantage, but as the true principle of ventilation is to have an opening for the exit of the contaminated air at the feet of each occupant of a room, or at the head of the bed of each patient in a hospital, it was soon observed that these shafts were too few and far between to make a very perfect arrangement. The necessity for providing for the escape of the foul air from the level of the floor in winter, so as to utilize the heat, was, after much opposition, finally established and officially acknowledged by the government officers. Then arrangements were made for its introduction into the government hospitals in a more perfect manner. I believe in no case, however, was it so fully carried out as to place a ventilating flue between each bed, but in some they were arranged, as shown (marked V) in the accompanying plans between every other two beds. These flues were carried together and extended through the ridge of the roof and capped as an Emerson ventilator; the opening into the large flue, extending to just below the ceiling, was closed in winter at all times, excepting when the room was too warm. This was for the exhaust, but of no less importance was the supply.