To show the importance of this, Simpson quotes a well-known fact, but one seldom thought of when we look at the brick walls of our houses. An ordinary well-baked brick, which is 9 in. long, 4½ in. broad, and 2½ in. deep, though apparently solid, is not really so. It contains innumerable minute spaces through which air may pass, and into which water may enter; and when it is soaked in the latter, and all the air is driven out, it will contain nearly 16 oz. (the old pint) of water. If one brick will retain in its pores so large a quantity, it is easy to see that a large wall may hold what most people would at first think an incredible amount. As Dr. de Chaumont says, “A cottage wall only 16 ft. long by 8 ft. high, and only one brick thick, might hold 46 gallons of water!” Walls may be made damp not only by water rising in them, but by rain driving against them, and by water running down from the roof in consequence of the stoppage of a rain-water pipe. The latter cause is simple and easily remedied, but the former is far too frequent in cheaply-built houses. It may be prevented by having cavity walls, as they are called--that is, a double wall with a space between. There are several advantages from this. The air space, besides helping to keep the inner wall dry, is a good non-conductor, and so the house is all the warmer. There are other methods which may be used in addition to this, as cementing, plastering, or covering with slates or boards. There is some difference of opinion as to the advantage or disadvantage of the walls of a house being porous, as bricks are when dry; and Prof. de Chaumont seems to think that in our climate the porosity of the walls is not a point we need trouble ourselves about maintaining. Still, in Simpson’s opinion, with the ordinary arrangements of houses as regards supply of air and ventilation, some porosity of the walls is desirable. Without the freest and most perfect ventilation, walls absolutely impervious to air, and therefore to water in a gaseous form, will almost always be more or less damp on the inside. [Illustration: 2. Damp Course and Area Wall.]