How are differences in the weight of the air to be utilised ? The first thing we have to consider is the condition of the air in an apartment The air that we breathe out, and the air given out by lights are hot, and consequently are light, and rise to the top; and for that reason the air in the upper part of an apartment is always more impure than in any other part. If you construct a closed chamber, and put lights into it, the top lights will go out first.

From this we see that the impure air we want to get rid of is chiefly at the top of the apartment, and the pure air low down.

Another thing to be considered is, that the air outside is colder than inside, and therefore heavier, and so whenever we make an opening from the outside to the inside the cold air will pour in. Now, those who first began to think about these matters, considered that as the foul warm, and at the top of the room, if they made an opening at the top the impure air would go out; but instead of that fresh air comes in, and you see why, because outside there is heavy air, and inside there is light air. and the exit for the air is np the chimney, so that the apartment is in precisely the same position as a box when you put it under water. There is another way in which air will come into a room, besides by openings specially constructed. It will come in through all openings, through the chinks of the windows, through the keyholea Air will also come into a room from the outside through the walls. A very considerable quantity of air penetrates into rooms through the walls, when the outside air is much colder than the inside. I may tell you that Professor Pettenkofer has shown that you can blow a candle out through a brick if you only concentrate the breath on one point; and so a large quantity of air is changed in rooms by the air passing through the walls when there is no contrivance to prevent it, and it is a fact that many rooms, when shut up in winter, are much better ventilated than in the summer, when the doors and windows are open, because at particular periods there is scarcely any movement of the air in summer, whereas in winter the cold air outside, and the warm air inside, form a current of air, and a large quantity passes through the walls. It remains, then, for us to consider how air is to be got into rooms so as not to produce draught, and how the air that has been used is to be drawn out. Cold air must be let in above people's heads, or it will produce a draught, and the practice has arisen of letting it in high up in a room. Now, if you make an opening through the wall of the room into the outer air very high up, the outer air will come in, cold and heavy, into the air which is lightest at the top of the room, and so it will fall down, just as cold water would, on to people's heads, and that clearly will not do. That is to say, then, that you cannot let cold air in low down, as it will get to your feet; and you cannot let it in high up, as it will fall on your head. What, then, is to be done ? You must let the air in so that it shall have a direction upwards, so that it shall come in like a fountain. There are several ways of doing that. Suppose you make an opening over the door through the wall into the outer air, and you put in front of that a piece of board slanting forward, air coining in strikes against that board, and is deflected upwards, so that it ascends as it comes in, and if you provide that board with what are called a pair of cheeks to prevent the air tumbling over at the sides, you have a very cheap way of letting air into the room. One precaution you should take, and that is, that you should not do that too high up in the room ; you should put it low down, only just sufficiently above people's heads. Now, many people do not like to go into a room where they see anything of the kind, and there are people, too, who directly anything of that kind catches their eye, think they feel a draught, but I can assure you that in a room provided with a contrivance of that kind there is no draught, it is pure fancy, and I would advise the plan as one worthy of general adoption; and I would suggest that you should conceal the piece of wood by hanging a picture in front, and then your nervous friends will not be annoyed. This contrivance has been brought under more control by an apparatus known as the Sherring-ham valve, which was invented some time ago, and answers its purpose very well indeed. It is a heavy iron valve, having a box which fits into a hole made in the wall; on the other side of this box is frequently placed a piece of perforated zinc, but an iron grating to keep birds out is all that is necessary, and is preferable. It has a heavy valve which is open when the box is in position, and this is a very good point about it. It can be shut by pulling a string if required, or it can be kept at any angle, because there is a weight which just balances it, and it has two cheeks to prevent the air falling over sideways. If this valve be used it should not be placed too high up in the room. A good many people are dissatisfied with it because they say the cold air falls down into the room, but the reason of that is that the apparatus has been placed too near to the ceiling, so that the air, after entering, immediately strikes the ceiling, and rebounds downwards into the room ; if placed a little above people's heads it does not cause a draught.

I have to mention next a very simple means by which air can be introduced into a room by means of the window, so as to go upwards, even in the coldest weather without anybody experiencing a draught. A piece of wood, about three inches high, which runs the whole width of the window, is placed underneath the lower sash, which is shut down upon it; this of course prevents the top and bottom sashes fitting in the centre of the window, and an opening is left, so that air comes in and goes upwards. It has a small disadvantage, and that is, that there is no provision for the exclusion of blacks, but that has been to a certain extent rectified by one of Mr. Tobin's plans. He does not put a block underneath the lower sash ; he cuts little pieces out of the bottom piece of wood of the top sash, and the air comes in and goes straight up, and no one can see ' that the holes are there. You can even hold a candle in front of the window, and no effect whatever is shown upon the flame. Into these little holes he introduces a small box, and fills it with cotton wool, keeping the wool in its place by little threads passing from side to side. That wool filters the air so that no blacks can come in. You can also have some small lids fastened on to the top part of the bottom sash to turn over and cover the holes whenever you choose, so as to prevent air coming in at all. If cotton wool is used it must be changed pretty frequently, for it is astonishing how soon it gets dirty.