By this expansion it is increased in bulk but not in weight; and in consequence, 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, was its gravity; the moment that becomes altered, and it, in consequence, rendered specifically lighter than the surrounding particles, it ascends, and passes through them towards the ceiling. This is the course of operation of the principle; the effect of it is this: - The moment this particle of air has moved away from what heated it, its place is taken by another, which, undergoing the same change, passes off in like manner, having its place taken by a 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 was 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 off the communication) poured, in one leg, quicksilver, 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 is very evident that the superior weight of the column of quicksilver would cause 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, till the contents of the two legs counterbalanced each other.
Now, this is an illustration 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 pressed against the thumb; and if a part of the ceiling be cut away, so 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, etc, 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, etc, 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 windows being, during cold weather, kept shut, the aperture 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 that are about either the doors and windows, or elsewhere around the building, and to introduce itself through them with a velocity so much greater than that at which it passes off by the ventilator, as will make up for the difference between the sizes of the cracks and crevices by which it enters, and that of the ventilation aperture.
This is the reason why drafts are experienced from the crevices of doors and windows: the heated and respired air passes off by the ventilator; to make up for what so passes off, fresh air flows into the bottom of the building, and as, when the weather is cold enough to make us shut the doors and windows, ingress by a duct equal to that of egress is prevented, to make up, by the rate at which it enters, for the difference in the sizes of the apertures of admission and emission, the air that finds its way in through cracks and crevices, enters with so great a velocity as to cause the chilling currents we experience.
Instead of suffering ventilation to take place at the pleasure of the air, I restrict and regulate it thus: - I first have the windows of the place nailed down, to prevent them from being ever opened; I then have the joints and crevices, both of these windows and of the room in general, so filled with putty, or so treated with any kind of lute or luting, that will answer the purpose, as shall prevent their becoming channels through which drafts or currents may find their way either into or out of the place. I then have the door-ways arranged thus:-Removing the present doors, the door-way is made six feet wide, by about the same height, and into it is fitted a cylinder (of wood or metal) closed at both ends, and placed upright on one of them, so as to appear somewhat like a cask built into the wall. Through the side of this cylinder I have two apertures cut, each about four feet wide, by the height of the cylinder inside its ends; which apertures are opposite, the middle of each being in the line of the centre of the cylinder, so as to leave a way of about four feet wide, right through the middle of it into the place, as shewn above, where the cylinder is represented placed in the wall, with the apertures in it.