The question of ventilation is so all-important, so much doubt still seems to exist as to the comparative merits of natural and artificial ventilation, so much has been said in favour of the latter, because it is seen adopted in the finest hospital in the world, the Lariboisière, that I have added a few practical remarks upon this system. The conclusion is, that even at the Lariboisière, without opening the windows occasionally, and especially in the morning, real freshness is never obtained in the wards, and that, therefore, if there must be artificial ventilation, that is the best which allows the most of the windows being opened. On the men’s side, Thomas’ system, or that of injecting air at given points, by means of two high-pressure steam engines, each working a large fan, is adopted. On the female side, Duvoir’s system affords ventilation by extracting air at given points. In both, a number of shafts and openings provide for the exit of the air. Persons at the Lariboisière Hospital, who ought to be good judges, including foremost the Director, an experienced and able administrator, consider the ventilation on the male side the most expensive and the best, both for day and night, being the coolest in summer and the warmest in winter. In winter two great advantages are assigned to this side: first, the heating being provided by the Hospital, the wards are warmable at pleasure; whereas that of the female side is provided by the contractor at a fixed degree of heat, which, in extra cold weather, is augmented: from 15° to 18° C are the usual limits of the winter temperature on the female side, while that on the male side is usually several degrees higher. Secondly, the system of heating on the male side permits the windows to be opened: while on the female side objections are often made to opening the windows in winter, which it is alleged interferes with the heating. _Per contrà._ How difficult it is to learn any facts by hearsay we know. Other persons who ought to be good judges think the male side apt to be too warm in winter, especially at night, and consider the temperature on the female side quite sufficient. Both sides are ventilated both by the windows and by the machinery by day in summer. Both sides are ventilated, each by its own different machinery, by night in summer, except that, exceptionally on hot summer nights, a window is opened two or three times in the night, or five minutes every hour. Both sides are considered to be ventilated in winter mainly by the machinery by day; and both sides are entirely ventilated in winter by the machinery by night.