1868.] Stained Glass. 153 tions, necessitate corresponding modifi- cations of our arrangements for venti- lation and warming. We must, there- fore, in the outset, make up our minds, that we shall have greatly to depend on constant individual care and intelligent thought, for the proper management of any efficient system of ventilation ; and, that it is quite impossible to have an automatic arrangement, that will fully accomplish the object desired. Of course, if you were to choose for a dwelling the centre of one of the PjTamids of Egypt, or the bottom of a mine, one thousand feet below the sur- face of the earth, then the troubles from the constant variations of the external atmosphere would be in a great meas- ure overcome. But those who find it more convenient to live in their paste-board houses, above ground, must provide the means of counteracting the inconveniences arising therefrom. This can be done easily, because, as many of our young men know, than a canvas tent with a flue pass- ing under it — so as to keep the floor well warmed, and with the proper intro- duction of warmed air to supply the necessary ventilation, — no more pleas- ant or comfortable room need be desired, even in severe winter weather. The thorough warming of the floor is the special point of excellence in this arrangement; and this must be care- fully attended to, in all habitations, where eomfort and health are desired. There is frequently much discussion, among young students of ventilation, as to whether the breath from the lungs falls to the floor, or rises to the ceiling. In practice, however, this is of very nttle importance, as there are other con- ditions or requirements that entirely overrule it. The great controlling prin- ciple is heat. This determines the open- ing or closing of all ventilating arrange- ments. The variations of temperature of the inflowing air, which we must assume to be the fresh air, have much more influence on the rising or falling of the impure air, than the mere differ- ence of its specific gravity. For instance, we take an ordinary room in winter, wherein the tempera- ture is maintained at 10°. Now sup- pose the air, by which that room is warmed, is flowing in at a temperature of 150° ; aud, this, much of the time, is the only fresh air entering the room. This would rise, in three or four seconds, to the ceiling ; and spread out, above all the air that had been longer in the room, whether more impure or not. In this case, if there were an opening directly at the ceiling, all the fresh and warmed air would there directly flow out, leaving the main part of the room cold and foul. But - now, suppose we open a door into an adjoining hall, the temperature of which is 40°. The cold air of the hall would at once rush into the room, at the bottom of the door, flowing under the warmed and perhaps foul air of the room, pressing it up towards the ceiling. Here, then, the fresher air would be at the bottom : or, if the external air was 25° above zero, all the fresh air coming through doors and windows would fall to the floor, more decisively still. Thus we see, that the conditions in which it would be exactly correct to have the exits, for ventilation, either at the top or bottom, are liable to be changed twenty times in a single minute. There are other important disturbing causes, such as the combustion of gas ; the very deleterious products of which rise directly to the ceiling, while very hot, but, if not' taken out of the room at once, are liable to fall to the floor again, from their greater specific gravity, or are sometimes crowded down, by the warmer fresh air from the heating appa- ratus. Here, again, we see the great inconvenience, to say nothing of the unhealthfulness, of exclusively warming a room, by circulating wanned air. But where the door and the exterior walls are heated, the fresh air may come in, a little cooler, than the contained air
