44 Mr Ti'edgold on the Principles and Practice 
evil. But, while the more extended benefit of open streets, and 
the free access of currents of fresh air, must be left to the care 
of public bodies, it is in the power of individuals to increase and 
improve the ventilation of their own dwellings. 
I have already noticed, that the air which is given out in res- 
piration is lighter than common air of the same temperature ; 
and that, being of a greater temperature than common air, it 
ascends as soon as it is expelled from the lungs : hence its pro- 
per outlet is at the upper part of a room ; but, in some cases, 
the same opening will give admission to a stream of cold air, 
unless it be of a peculiar construction. To avoid this defect, 
there should be a free supply of fresh air to the lower part of 
the room, and the openings contrived so that their action shall 
not be interrupted by winds. It will be found an advantage to 
let the ascending air flow into the space between the ceiling and 
roof. We will suppose a case where the vitiated air passes im- 
mediately through the ceiling into the space in the roof, as shewn 
in Fig. 3. Plate II., where its course is indicated by the dotted 
lines, the apertures through which it ascends being concealed 
by ornamental plates, AA, placed at a little distance below them. 
If cold air be forced in at the top or otherwise, it will occupy 
the lower part of the space, as at BBB, and cannot make its 
way into the tubes DD, unless it be in greater quantity than 
fills the space above the level of the tops of these tubes. The 
top C should not be longer than is required for the intended 
purpose, and the greater height it has the better, but it should 
not, in any case, be higher than the chimneys of the building, 
as it may cause them to smoke. Where a room is required to 
be ventilated, and is not next the roof, the air-tube should be 
got by the most favourable direction inta the space in the roof. 
In all cases the apertures should be provided with registers that 
can be opened or closed at pleasure. The most simple is con- 
structed in the same manner as the throttle valve of a steam en- 
gine. It consists of a plate A, fixed on an axis in some part of 
the air-tube, and is represented in Fig. 1. It should not be 
made to move too easily, in order that it may stand at any open- 
ing at which it is set. 
The apertures for admitting fresh air ought to be abundant- 
ly large, and covered with wire-gause, that rapid currents may 
