486. The presence of moisture being one of the conditions which appear to be necessary for the decomposition of organic substances, it is desirable that the neighbourhood of buildings where timber is used should be well drained, which not only prevents decay, but tends to preserve the health of the occupants.

Drains should be made water-tight when they come near to walls, particularly those built of bricks, which readily absorb moisture, and conduct it to a considerable distance. Earth or soil liable to become damp should never bo permitted to rest against the walls of buildings; and when the lowest or basement story is sunk, the building should be surrounded by a dry or open area.

* ' Japan,' vol. ii., p. 127.

487. The moisture should be prevented rising from the foundation by interposing a layer of some impervious substance through the thickness of the wall, at a height of several inches above the ground. Sheets of lead or copper have been used for that purpose, but as they are expensive, two or more courses of slates that do not readily absorb moisture, laid in Portland, Roman, or some kind of bituminous cement, have been substituted, with a layer of the cement spread over the upper surface.

A very perfect damp-proof course is formed by spreading a thin layer of asphalte - say about 3/8 or 1/2 inch thick - throughout the thickness of the wall: the best position for it is on the top of the course which is immediately under the wood sleepers that support the ground joists.

A very good description of damp-proof course is that formed of glazed earthenware, known as "Taylor's Damp-proof Course," when properly laid in a cement that is impervious to moisture.

Whatever kind of damp-proof course be used, care should be taken to prevent its being broken or fractured by the weight of the work placed above it.

Rendering or weather-slating the outside vertical surface of walls is also a good expedient to keep out damp, and so preserve any wood that may be in contact with the masonry of the building.

488. After a building is covered in, and before the doors and sashes are hung, the walls and principal timbers should be left for some time to dry, which will also enable the latter to settle to their proper bearings, and so prevent cracks from appearing in the plastered ceilings and partitions, the execution of which should be deferred until after the timbers have ceased to shrink or settle.

Timber should not, however, be left to season until after it is fixed, but the fact of its imbibing a certain quantity of moisture from the damp walls of the building (which causes it to swell), renders some amount of shrinkage afterwards, on drying, unavoidable.

A considerable time should also be permitted to elapse after the plastering is finished to allow of the work becoming dry again, before the floors, skirtings, and other joiner's work be fixed.

"When a building has been thoroughly dried at first, openings for the admission of fresh air are not so much required as where the precautions against fresh accessions of moisture have been neglected. Indeed, such openings in the walls of a building, when sufficiently large to be of use in preserving the woodwork, are frequently a source of discomfort to the occupants, from the difficulty of regulating the supply of air which passes through them. In many situations - as under the floors of a basement story, particularly when the openings are small - it is difficult to obtain a sufficient current of air to carry off any considerable quantity of moisture; consequently, we find that the timber of floors in such situations is more liable to decay than that in roofs. And Mr. Papworth observes: "Should the air absorb less moisture from the fungus than the timber affords to its vegetation, the air will then increase the disease, and draw into fuller growth the fungus it has not the power to destroy; but if dry air be admitted in a quantity adequate to cause that absorption, it will necessarily exhaust and destroy the fungus." * From which it would appear that a small quantity of air may do more harm than good.

* ' Essay on the Causes of the Dry Rot,' p. 43.

489. In floors next the ground the access of damp should be prevented as much as possible by careful drainage and the removal of all vegetable mould from the surface, and, where possible, a layer of concrete or a considerable thickness of dry materials, such as brickbats, dry ashes, etc, but not lime, should be laid under the floor, and over these a coat of smith's ashes, or of pyrites, where they can be procured. The timber for ground-joists and sleepers should be particularly well seasoned; and it is advisable to cut off all connection between the wooden ground-floors and the other woodwork of he building.

490. Well-seasoned timber is likely to last for a long time unless placed in situations that are favourable to putrefac-tion; and as such situations cannot always be avoided, various remedies have been suggested with the view of retarding the process of decay. None of them have met with more than a partial success, and many of them have been complete failures.

491. Oil paint is most commonly used for the protection of timber when exposed to the weather. It forms an impervious coat on the surface, and prevents the moisture from penetrating to the wood; but if the timber has not been previously well seasoned, the application of paint will rather promote than retard the process of decay, by preventing the evaporation of the juices of the wood, which ferment and putrefy.

For timber that is not exposed to the weather the utility of paint is somewhat doubtful; in joiners' work, where it is usually cut into thin slabs or small scantlings, which ensures it a better seasoning, paint may be of some use in preventing the wood from cracking, as it retards the evaporation of any moisturc that may have remained in it.

Wood used in out-door work should have those parts painted only where moisture is likely to find a lodgment, and all shakes or cracks and joints should be filled up with white-lead ground in oil, or oil putty, previous to being painted over.

Any of the common paints will answer for the preservation of wood if mixed with good linseed or nut oil, and for work exposed to the weather they should be renewed about every three or four years. Sand shaken rather plentifully upon paint while wet renders it much more durable. To preserve timber exposed to the weather, Chapman recommended a paint made of sub-sulphate of iron (the refuse of the copperas pans), ground up with any cheap oil, and rendered thin with coal-tar oil in which a little pitch had been dissolved.*