472 Transactions, — Miscellaneous. 



revolving from west to east, and, as the atmosphere is not 

 carried round at the same speed, there is a tendency to pro- 

 duce a current in the opposite direction — i.e., from the east. 

 These two movements combined cause the air-currents to take 

 a middle course, and to form in the Northern Hemisphere the 

 "north-east trades" and in the Southern the "south-east 

 trades." 



The only part of this illustration which will help our pre- 

 sent investigation is the first part — viz., that which shows 

 (1) that warm air will always rise, and (2) that cold air 

 will readily flow in to take its place. The law which governs 

 this action is simply that which causes water to seek its own 

 level. The reason of the latter tendency is that, when water 

 and air together are drawn downwards upon the earth's sur- 

 face by the attraction of gravitation, the water, having a 

 greater specific gravity than air, is drawn to the lowest parts, 

 and displaces the air from them. But, if quicksilver was also 

 in the race, it would as speedily displace the water. This 

 law, then, is universal in its operation, and causes all fluids to 

 range themselves in horizontal layers according to their specific 

 gravities — the lighter at the top and the heavier at the bottom. 

 Now, warm air is of less specific gravity than cool air, and 

 therefore floats above the latter, and, for all purposes with 

 which ventilation has to do, acts just as if it was a distinct 

 fluid. 



Having realised that the law governing our problem is 

 simply that of gravitation, let us apply it to the conditions 

 which we know exist in connection with a house. We are all 

 conscious that the average temperature of the ah inside a 

 house is greater than of that which is outside. It must follow 

 that there is a constant effort being made by the inner air to 

 float out at the higher openings, and by the outer air to flow 

 in at every opening, whether high or loio. A simple illustra- 

 tion will make this action clear : Supposing it to be possible 

 to lift a house bodily, and to immerse it suddenly in a lake of 

 water, the result would be that at all the joints of doors, 

 windows, and slates, under the eaves, and down the chimneys 

 water would commence to flow in. The air would at the same 

 time begin to escape and to bubble up to the surface. A little 

 consideration will show, however, that it would not come out 

 of the house at every opening at which the water was enter- 

 ing. As soon as the water rose on the inside of the house 

 above any opening, that opening would cease to emit air, 

 because it would always be impelled upwards and never down- 

 wards. If there were no outlets from the rooms above the 

 tops of widows or doors a quantity of air would be imprisoned 

 in them as in a diving-bell. To make this illustration apply 

 to ventilation we have only to substitute outer (or cool) air 



