We will now place another tube over the lower opening (Fig. 12). Just see what a wonderful effect that has! Here is the air rushing down this short flue and up the two cold ones. We called those two first pipes cold, but our ideas of heat and cold are simply comparative; everything is warm, or has heat in it. Perhaps some of us think there is not much heat in the air when it comes whistling around our ears 15° or 20° below zero; but the cold rigid chemist will still extract many degrees of heat from that. We must, therefore, remember that absolute temperature has nothing to do with the air passing up or down a flue—it is simply comparative temperature.
Let me show you one more experiment. Here are two tubes we have had heated; as you see, the smoke rushes up them rapidly. But now we will add this third one (Fig. 13), which reverses the current at once. The two first are hot, taking the temperature of the room as the standard, but the third one is still hotter.
The form of a flue has but little to do with the draught; the height has a slight influence, but bear in mind constantly that the great moving power in all flues is the variation of temperature.
Now, let us make a practical application of this principle.
Wait a moment: just let us lay this one aside, but not forget it, as we shall want to refer to it in a few moments, and try another experiment which has some bearing upon the subject.
I have here a tube just one foot square and two feet long, and one foot from the bottom there is what we will suppose to be an air-tight piston that can be moved without friction. Now, suppose we heat that air 490° (for the sake of easy remembering, say 500°); this would just double its volume—it would then be two cubic feet in size instead of one.