To change the air of a building two things are primarily essential: (1) an Air inlet, and (2) an air-out.

This may appear so obvious a requirement that no more need be said there-on, but although it may in the abstract be generally known, it seems to be only partially recognised and is frequently ignored in practice.

In opening a window to obtain more air, as it is termed, in an apartment, is it always recognised how that which was previously there finds exit? Air entering by the window is felt, or is known to be coming in by the movement of objects in its vicinity, but inasmuch as the room was not previously void of air, and as other air could not enter without displacing that which was previously there. some means of escape there must be. If, then, by opening a window appreciable change of air in an apartment takes place, several considerations arise.

I. A contrivance, i.e. the window, made to open, had been provided, partly at least (wo may presume) for the purpose of ventilation. 2 A Personal act was performed, viz. the opening of the window.

There must have been some other outlet or inlet communicating directly or eventually with the outer air, although neither may be well-defined nor clearly visible 4 According to the size and position of the openings, both for inlet and outlet, a given change of air in the apartment would take place under similar conditions.

5. If we presume that the apartment was previously occupied, there must have been some change of air going on before the window was opened.

6. If this incoming air entered directly from the outside, where was the inlet, or more proltably inlets? Were they in proximity to anything by which the air might be contaminated, such as drain-gullies or accmutilations of refuse, the household midden or dust-hole, or rain-water-pipes with open joints and improperly connected with the soil-dmin

7. If the air had previously traversed some portion of the building, where did it first enter, what direction did it take, and what may it have passed on the we If pure on entering the building, could it have been contaminated on its way to the apartment?

8. Was the change effected by a force acting from without, or by a force acting from within? If from without, was it a propelling force, which drove the air in and so displaced that which preceded it; or was it a suctional force, which extracted that from within and gave place for other air to enter? If from within, what force could have been brought into action which would cause either a forcing-out or a suck-ing-in of air?

9. Having presumed that the opening of the window was necessitated for the greater comfort of the occupants of the room, it follows that, previous thereto, change of air was insufficient, or that the source from whence it was drawn was not a pure one.

10. Did the opening of the window lessen or completely atop the entrance of air from other sources? Was the air entering at the open window uncontaminated? What was its temperature and rate of movement \

11. Could the occupants continue in the room with comfort while the window was open, or would other discomforts, such as draughts or excessive cold, compel them to shut it again?

12. If after an interval the window was again closed, and the occupantexperienced greater comfort for a time than they did before it was opened, what deductions must be drawn'.

13. Might not the regulation of the window-opening from the first have secured comfort to the occupants? If not, could other openings in any other part of the room have provided the requisite change <>f air, without discomfort, at the particular time and under similar circumstances?

14. If so, would a change of conditions, or a different state of the outer atmosphere, necessitate further regulation, or the closing of such openings?

15. In order to secure comfort for the occupants, was there a fire burnting, or some other means of raising the temperature of the room?

16. What was the cubical capacity of the room?

17. What were the number of the occupants, and their occupation ? Although the foregoing list by no means exhausts the questions which may arise even under such ordinary circumstances as have been supposed, it is sufficient to indicate the following general principles: -

1. That, after ascertaining the possibility of procuring reasonably pure air from the outside, constant care will have to be exercised in order that it may not be contaminated either just before or while entering the building, or after having entered.

2. That openings must be provided for its entrance and for its exit, with means for regulating either one or both.

3. That the regulation of such openings requires frequent and intelligent attention.

4 That when change of air is brought about within a room it is by means forces which, if natural, are constantly varying, and therefore when such natural forces are employed, the rate at which change of air takes place must necessarily vary.

A mechanical power may be employed, which would be more regular in action than the natural power of wind, or than the force exercised by the air within and that without being of varying density, caused by difference of temperature, but up to the present time only a few ordinary dwellings have been ventilated mechanical means, and there can be little doubt that only in exceptional cases can mechanical means be employed for economically ventilating isolated houses of moderate dimensions. For public halls, churches, libraries, hospitals, schools, factories, and other buildings cut ted for public purposes, particularly those in which many people congregate, ventilation by mechanical means alone can be depended upon to give constant and satisfactory results. Its use may nlso with advantage in time be extended to dwellings built compactly together or in flats. The principles of mechanical ventilation, however, require the utmost kuowledge and care, and their application should only be attempted by those who have made a thorough study of the subject.