Here we have our little glass house again. We will take the roof off and put a pretty large family in it--I mean large in numbers, if not in size. You may call it a school, or public meeting, or church, or whatever you please. Suppose, for illustration, we call it a church, and we will call this larger light in this end the minister speaking to the congregation. You see, the lights are a good deal agitated, and flare around a good deal. There is a rush of air down at this end, and, as it becomes heated, it rises at the other. Let us cover about one-half of this up. Now see what a rush of air there is _down_ these flues, instead of _up_ them, as there ought to be. Here, you see, the main body of the building, though much shorter than the flues, forms the heated leg of the syphon; and you may thus recognize why many of the ventilating flues, put in the cold outside walls of many of our large buildings, persist in working the wrong way, and cold air blows down there, instead of the foul air going up. But there seems to be too much draught. Let us put the roof on. Ah, that is better; but, then, what a draught there is down this chimney-flue. Call the sexton, and have that stopped up quickly, or those sitting near there will soon catch their death of cold, and will never come here again. You see, however, they shine very brightly, notwithstanding all the draught, but there, now, it is all closed up as snugly as the most fashionable church in town. See how quiet and peacefully they burn now. Ah, there is one just gone to sleep. You must excuse him, he probably was up most of the night with a sick child. And there goes another. I think he must have been very busy for the last week settling up his last year's accounts. Just see, they are going to sleep so fast, I don't think we can pretend to give excuses for them all. And, now, is not that a brilliant congregation to be preaching to? Everyone dead asleep excepting the preacher himself, and I suspect he feels stupid enough to go to sleep, but it would not look well; and he has to tax his energies so severely he will hardly get over it, so as to be good for anything for the balance of the week. You may think this an exaggerated representation of the real facts. Do not deceive yourselves. A few months since I was requested by one of the congregation to visit a building within a few minutes' walk of this place, and see if there was not some defect in the ventilation. The gentleman stated to me that he sometimes attended the class-meeting, and would be glad to go oftener, but it was held in the basement story, and it was quite impossible for him to keep awake, as he had to get up and go out two or three times during the evening, to get a little fresh air, or he could not keep awake.