As the employment of mechanical means for ventilating single dwellings and is likely to be, of rane occurrence in this country at least for many years to come it is not proposed to enter further into the subject at present, and, therefore, the forces which Nature supplies will now be considered. It cannot be too strongly insisted that change of air within a building is invariably brought about by the exercise of a power capable of being ascertained. This power can be traced back to the effects of heat. either of the sun or arising from the combustion of fuel, and that the effects of heat are made operative by the force of gravitation, which, apart from the employment of mechanical means, may justly be regarded as the active agent by which change of air is brought about naturally to assist ventilation. Knowledge of this fact simplifies the whole subject, and explains much that otherwise may appear complex. There are certain other natural properties of substances which exercise influence upon the atmosphere, and to some .extent regulate air-movements, but they are compara-tively insignificant compared with the force of gravitation.

A property of the atmosphere, which is, however, common to most gases, is that heat-rays pass through it without materially raising its temperature. and yet when air comes in contact with either solids or liquids, the temperature of which differs from that of the air, the latter will either absorb or give up heat; this property exercises a powerful influence in creating movements in the atmosphere, and consequently upon the ventilation of our houses.

When its temperature is raised, air expands, and becomes, hulk for hulk, lighter than air at a lower temperature, the result being that cold air falls by the power of gravitation, and in doing so causes the lighter air to ascend. As this process is going on at all times to a greater or less degree, throughout the whole surface of our globe, movements of air, constantly varying in force, are set up. In some districts this movement, brought about by the great heat of the sun, is so rapid as to cause violent storms, hut generally the action is more regular, resulting in what are known as trade-winds; yet, as day succeeds night, and many local conditions induce variations in the earth's temperature in different places at the same time, even these comparatively regular trade-winds are variously affected, resulting in what are popularly known as changes in the weather.

To a very large extent, it is upon these most variable movements of the outer atmosphere that we must depend for securing ventilation within our homes. The forces they exercise may be employed either for propulsion or for extraction, or for both at one time. In warm weather, when it is possible to throw open doors and windows freely, the propelling force of wind is almost exclusively relied on to obtain change of air within a building, but when doors and window are kept closed, the suctional influence of the wind, passing over the open chimney-pots and air-openings, has a considerable influence upon change of air within.

In addition to these forces from without, when houses are warmed either by open fires or any other means, additional forces are developed within, which may either assist those outside in causing change of air within, or may be antagonistic thereto. Smoky chimneys are evidence of this antagonism. The forces set up by the employment of heat within a building are caused by the interior atmosphere taking a higher temperature than that without, whereby it expands, and, being lighter than an equal volume of the air outside, is forced upwards by the colder and denser air descending.

Just as the effects of variation in temperature outside produce movements of air, which may be simply local or of vast extent, in the same way but in a lesser degree will varying temperatures within a house or even an apartment cause local or general movements in the atmosphere within. Such local movements must not be confounded with actual change of air, because local movement or circulation within a building ii not sufficient to bring about that change of air which exercises the purifying effect required for the purposes of good ventilation.

The British home is the stronghold of the open fireplace, and this, on account of its cheeriness and its assistance in ventilating the house, is likely for long to find an honoured place A valuable amount of ventilation is secured, not alone by the presence of the tire, but to a large extent because of the necessity for a smoke-flue or opening to the outer air of adequate size. Many people overlook this fact, and when no fire is placed in the grate, most unreasonably close down the register. stuff something up the flue, or place a screen over the grate-opening, and in so doing prevent that free change of air in the apartment, which almost invariably goes on so long as the flue remains open. Such practices cannot be too rigorously condemned, for whether there be a fire in the grate or not. the flue from every room should be regarded as a most important outlet for air. the exit of which will alone permit other air to enter.

Because of the more general tendency of up-draught in flues, there is a belief that they exercise a suctional influence, but the diet is that, wherever such suctional effect is noticeable, there is an actual force to cause it, the flues being simply channels by which the resulting movement of air is made apparent.

In like manner it has been thought that with flues of varying height, connected at the base, the longer one will exercise a syphonic influence. When a downward current is observed in the shorter flue, careful examination will demonstrate that it is the result of some disturbing cause in the atmosphere itself or of some force applied, without which the flues would of themselves be quite incapable of influencing the movement It wouldl be as reasonable to expect water to circulate within a syphon plunged into a vessel of water. The material form of the syphon or of the flues cannot, of itself, exert any influence which would cause a movement therein. Nevertheless it is a fact in practice, that what appears to be a syphonic action does frequently take place where there are flues of varying lengths in air-connection at the base, but it is the result either of variation in temperature, or the suctional or propelling force of wind, or of mechanical power, and it would be quite easy to apply any one of such powers to cause the air to ascend the shorter flue and descend the longer one, although under ordinary circumstances the reverse will take place.