96 The Library. long as the wind is blowing into the mouth of an inlet, it may be assumed that fresh air will find its way into the building in a reasonable amount. But if the atmosphere is stagnant, or the direction of the wind is such as to carry it away from or past the inlets, then the air of the building also becomes stagnant and is soon vitiated. This objection is obviated by the introduction of machinery, which working, irrespective of varying external con- ditions, ensures a constant supply of fresh air in all circumstances. But, as I have said, the system adopted in the Aberdeen Public Library, is not only mechanical it is mechanical in a special way, operating by a process of impulsion as distinguished from one of extraction. The difference between the two methods is considerable, and not the less so is that between the results as affecting health and cleanliness. Mechanical ventilation by extraction proceeds on the principle that given a vitiated atmosphere in a room, the best way to purify it is to induce an upward current of air by means of a fan or other contrivance. In this way the foul air is extracted, and is replaced by fresh air, which finds its way in by various channels provided for the purpose. But by this method not only are dust and other obnoxious matter carried into the room, but the air is sucked in at windows and other places where it is not wanted. A more serious objection is that there is no proper diffusion of the incoming fresh air, owing to its tendency to pass directly through the room from the point of entry to the point of exit. Now with ventilation by impulsion, the case in this respect is altogether different. Here the fresh air which it supplies is forced into the room in such a way that it cannot fail to be thoroughly mixed with the air already there, from which in turn it is similarly expelled. To understand how this is done in the public library here, I would remind you that the building consists of three main floors devoted, exclusive of subsidiary rooms, to the reading-room, the lending department, and the reference department. Each of these measures about 75 feet by 45 feet, by 18 feet high, giving a cubical content to each of over 60,000 feet. For its supply of air each is dependent on what is forced into it through the channels provided, all others being as far as possible closed. There are no fireplaces, and the windows are sealed. On the basement floor, behind the reading-room, and a little
