The aspect and situation of a house may have a decided effect for better or worse upon its ventilation. If overshadowed by more lofty buildings, elevated ground or trees, free circulation of air around may be impeded, or the revivifying rays of the sun may be shut out. The position of a house with regard to others even of a similar class may likewise affect ventilation.

Wherever it is practicable, windows should be provided on more than one side of a room, it being better for the cheerfulness of the apartment, as well as for the purpose of ventilation, to distribute them than to have all the window-Space massed together. The literal meaning of the word window is "an opening for the wind", and although windows are now more generally regarded as means for admitting light to the interior of buildings, they ought nevertheless to be constructed so as to be capable of being easily opened, and should still be considered the principal means for securing change of air in an apartment. When there are more than one, on different sides, it is possible to utilize outer movement of the air to cause change within, quite independently of other openings, and although. when the room is occupied, it may not at all times be comfortable to permit of such rapid movement of air within as may frequently be induced by the opening of windows, then are times during which room are unoccupied, when it may be allowed with great advantage. Hence the advisability of carefully considering, during the planning of a house, the relative positions of windows and doors with a view to securing a thorough change of air throughout the building or in individual rooms at convenient times.

Bay-windows arc particularly adapted to securing comfortable ventilation in hot weather, because, by a considerate adjustment of curtains and blinds, it is generally possible to have the window open on one side at least of the bay, and yet to shut out the glare and heat on the other sides.

Detached houses should be situated in reference to their surroundings, so as to facilitate free circulation of air around, and the doors and windows should be so placed that advantage may be taken of the best aspect for the several apartments, and that, when desired, windows may be opened freely to catch the most favourable breezes. Through ventilation ought then to be secured, for, when fresh air surrounds a building, it is possible, with a little care in the opening of windows, to obtain a movement of air through the building at almost any time. If the wind be strong outside, a moderate opening of windows will suffice, and, even when the air outside is sultry, by freely opening them on opposite sides refreshing movement may generally be secured within, brought about by the varying temperature on one side of the building and the other.

Houses in rows or terraces are not bo readily ventilated at all times as detached houses, because air has not free access all round them. Much, however, may be done in their planning bo as to secure a reasonable amount of through ventilation, because the difference in temperature between one side of the houses and the other at the same time will, at most hours of the day, cause a movement of air through them if windows be judiciously opened.

Houses in courts and alleys are more difficult to ventilate properly; conse-quently their erection should not be permitted. Fortunately they are now-prohibited in all well-regulated localities.

Back-to-back houses are even more objectionable, and should be condemned.

Lofty houses, especially if built close together, are not only difficult to ventilate, but, in addition, they overshadow so much ground surface or other buildings in their vicinity, that purity of air around, and consequently good ventilation within, cannot readily be maintained. In other words. Overcrowding of a given surface of ground must be avoided if efficient ventilation of dwellings is to be secured.

If buildings are erected of impervious materials, change of air within can only be adequately effected by providing definite entrances and exits. This of itself might be an advantage if these entrances and exits could be so arranged as to cause the incoming air to disseminate through every portion of the building, because then we could the more readily regulate the quantity of air which should enter and leave the building as well as determine whence it came. The ditfi-culties, however, of securing suitable appliances, and that frequent personal attention which would be necessitated by the ever-changing conditions of the external atmosphere, lead me to believe that such buildings are not likely to be the best ventilated. Moreover, there are positive disadvantages in the use of impervious materials, because, as a rule, such materials do not retain heat; they may become quickly heated and as quickly cool; consequently variations in the temperature outside soon affect the condition of the air within. There are, however, other impervious materials which do not readily absorb heat, but reflect it. and are therefore cold to the touch. In either of these cases, there is always a tendency, when the outer atmosphere is colder than that within, for moisture and volatile substances, as well as organic matter, to be condensed upon the impervious materials. The author has frequently noticed that in rooms the wails of which arc covered with varnished paper, and where the floors are impervious, it is most difficult to obtain ventilation without draughts.

Some may suppose that the pores of pervious walls would become contaminated by the passagc of air carrying with it impurities. This might occur in localities where the atmosphere was very impure, but then it would be equally injurious to admit the air by a window or other opening. Under ordinary conditions, impurities in the air would be deposited near the surface, and would be subject to the generally purifying effects of the outer atmosphere.