Ventilation from below is effected by means of airchannels and grated openings in the floor, but it is disagreeable on account of the draught thrown upon the legs of persons in the room. A better method is to have the skirting boards of the room arranged so that spaces for the transmission of fresh air shall exist between them and the walls.

Arnot's valve ventilator, which is inserted into the chimney at some distance above the fireplace, is valuable because it is self-regulating, as all ventilators should be as much as possible.

Several methods of ventilating from the roof are employed in factories, stables, and other buildings. One method is interesting because it depends upon a curious property of the atmosphere. It consists in inserting, perpendicularly, into the roof a tube, which is divided longitudinally by a partition. One extremity of the tube communicates with the outer air, and the other with the room to be ventilated. It is found that the foul air passes out through one part of a tube so divided, while the fresh air passes in through the other part. A tube inserted within another tube, with a space between them, is sometimes substituted for the divided tube. But the objection to both methods is, that at the ceiling the fresh current in its descent into the room mixes with the vitiated ascending current.

We have now to describe one of the best and simplest modes of ventilating ordinary rooms with which we are acquainted. It is one equally applicable in winter and in summer, because all draught is avoided; for even if a window be opened at the top, a downward draught is frequently felt, and in rainy weather it is often impossible to keep the window open. The present plan is applicable in all kinds of weather, and would be perfect if the ventilation could be effected nearer to the ceiling.

As it can be applied at an expense of a few pence, and as no unsightly appearance is made, it is equally applicable to the cottage and to the mansion. A reference to the woodcut will greatly assist our description. A piece of wood (a) an inch or more in thickness, three inches wide, and exactly as long as the breadth of the window through which ventilation is to be established, is to be prepared. Let the sash be now raised, and let the slip of wood be placed upon the sill of the window the sash is then to be drawn down closely upon the slip of wood (A). If the slip has been well fitted-and the fitting may be made more complete by adapting it to the grooves in the sash and its frame, if any exist no draught will be experienced in consequence of the displacement of the sash at this part. The effect of such an arrangement is, however, to cause a

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separation between the bars of the sashes at D. By this means perpendicular currents of air will be projected into the room between the glass in the upper and lower sashes and their respective bars, in the direction of the

arrows in the engraving, and other currents will pass outwards in the reverse direction, in a manner by which all inconvenience from draught will be avoided.

Supposing that two or more windows at opposite sides of a room are fitted in this manner, a very satisfactory ventilation will be provided. Owing to a difference in its equilibrium, the air will rush in on one side and rush out on the other side of the apartment. If the slips of wood are painted of the same colour as the windows themselves, they will attract little notice.

We cannot conclude the subject of ventilation without an appeal to clergymen, schoolmasters, and others who are in positions of authority. Immense good may be done by impressing upon the minds of those over whom they are placed, the vital importance of breathing pure air. Especially should this be instilled into the young. It forms as yet no essential part of a liberal education, that a man should be taught to understand the conditions upon which he lives, or how he should best preserve his health. Such knowledge is certainly not less important than most of the instruction he receives. Yet all the knowledge which concerns his physical existence is left to be picked up by chance, or to be gained by experience, an experience sometimes only obtained by the sacrifice of health. The subtle causes which vitiate the air we breathe, must, as we have seen, be sought out to be understood. And if this kind of knowledge is important to those who live in large and airy houses, how much more important is it to those

who pass their lives in humble cottages, and in the closely packed tenements of towns! How many headaches would be avoided-how many a pallid cheek would be tinged with the glow of health-how many drooping spirits would be roused to the enjoyment of life-how many sickly infants would be transformed into vigorous men and women, instead of being prematurely cut off by disease, were the simple facts universally known and acted upon, that no kind of stimulant is so permanently enlivening, no food more strengthening, than a proper supply of fresh air in our houses.

It is a pleasant reflection, that within the present century, owing to many causes, but chiefly to the advancement of science, longevity has greatly increased in this country. We feel assured that a very considerable increase is still to be effected by a more widely spread knowledge of the principles and practice of ventilation.