Notwithstanding this arrangement is calculated to render the atmosphere of crowded places more fit for respiration, it is productive of e painful and serious inconvenience to those persons who may be situated near to the apertures before mentioned, where the fresh air enters; they are thus exposed. as it were, to the action of a series of blow-pipes, and the consequences are, colds, asthma', and rheumatism', in abundance. To avoid drafts, and yet ventilate thoroughly, has hitherto been • found of difficult accomplishment. In " A letter to the Earl of Chichester, on the practicability of rendering those properties of air, which relate to caloric, applicable to new and important purposes, (1823,)" by Mr. John Valiance, of Brighton ; that gentleman has proposed a plan for warming and ventilating the Houses of Parliament, which, is admirably designed to obviate the difficulties just mentioned ; we shall, therefore, give it a place here in the author's own words ; although there are some mechanical difficulties to be overcome, before it can be rendered elegant and convenient; the means of effecting which, will, we trust, be ultimately accomplished.
"There are two principles which operate to alter the state of air, in any place where numbers of people convene. One of them affects it physically, and to a change of density, and is the cause of drafts and influxes of cold air; the other affects its chemically, and to • change of quality, as the medium by which the action of the lungs is rendered efficient to the preservation of life, and renders necessary, and indeed indispensable, the drafts and currents of which the first is the cause. The first of these occurs in every place in which air is heated; the other, only in those places in which it undergoes respiration. Now, it is the first only of them that falls under our consideration, when investigating the principle on which drafts take place; and the course of operation of this prin ciple is thus. If heat be communicated to a particle of air, a change takes place with respect to that particle in the following manner; it becomes expanded and increased in bulk, in some such way as may be conceived, by reference to the juvenile practice of holding • fiaccid bladder before the fire, to tighten and fill it up again, prior to using it as a football.
By this expansion it is increased in bulk but not in weight; and in conse quence, rises from among the other particles, and ascends towards the ceiling ; in the same way that a bladder, filled with air, would rise through, and swim at the top of others filled with water, were they thrown into the sea together; and, as the only circumstance which caused this particle to be where it chanced to be at the time this supposed heating took place, wu its gravity; the moment that becomes altered, and it, in consequence, rendered specifically lighter than the surroun&ng particles, it ascends, andthrough them towards the ceil ing. This is of operation of the principle; the effect of it is this :—The moment this its of air has moved away from what heated it, us place is taken by another, which, undergoing the same change, passes off in like manner, having its place taken by • third particle ; and this alternation continues all the while heat is communicated, be the communicator what it may, whether the human body, a stove, or any other method of heating.
This is the effect of air's being heated ; its physical state is altered, and it becomes specifically lighter than it wu before.
The consequence of its becoming lighter, may be conceived thus :— If a glass tube were taken, shaped in this manner, with a notch or crevice cut in it at A, to which notch or crevice a metallic slide were well and tightly fitted, so as to cut occasionally off the communication between the two legs ; if into this tube there were (when the slide was pushed in, so as to cut of the communication) poured, in one leg, and in the other water, and then, when both were full, (placing the thumb on the top of the leg that had the water in it, to keep it in,) the slide between the quicksilver and water were pulled out to let them press one against the other„it u very evident that the superior weight of the column of quick silver would came it to press the column of water upwards against the thumb, and that, were the thumb removed, the water would be driven up, and some of it forced out of the tube; and also, that the water would continue to rise, tilt the contents of the two legs counterbalanced each other. Now, this is as illus tration of what takes place in any building, whenever the air inside it is hotter than the air without.
Since, the external air being heavier than the internal air, the former so operates upon the latter, as to press it upwards against the ceiling, in the same manner that the water would be premed against the thumb ; and if a part of the ceiling be cut away, an as to open a means of emission similar to what the removal of the thumb permitted ; that is, if a ventilation aperture be opened in the ceiling, the superior weight of the cold external air will cause it to drive the lighter internal air up through that aperture, till the equilibrium becomes restored: and if, owing to the air on the inside being by the respiration, fire., of people convened in the building, kept constantly warmer and lighter than the external air, this equilibrium is prevented, and the difference between the external and interval atmosphere kept permanently up, the consequence then will be, that instead of a single and transitory emission, like that of the water from the tube, there will be a continuous emission of air through the ventilator, all the while the respirations, &c., of those who are assembled in the building keep up the difference. Now this is what takes place in all public places; and as, owing to the door and window* being, during cold weather, kept shut, the aper ture of admission (or channel by which the external air enters the building.) is rendered very much smaller than that of emission ; to make up for the difference thus caused between the apertures of admission and emission, the cold external air is obliged to make use of all the cracks and crevices are about either the doors and windows, or elsewhere around the building, and to intro duce itself through them with a velocity so much greater than that at which it ensues off by the ventilator, as will make up for the difference betweeti the sixes arks cracks and crevices by which it enters, and that of the ventilation aper lure.