Another formidable fungi, which attacks oak in ships, is the _Polyporus hybridus_ (the dry rot of our oak-built vessels). It is thus described by Berkeley: “White, mycelium thick, forming a dense membrane, or creeping branched strings, hymenium breaking up into areæ, pores long, slender, minute.” From the slow progress dry rot makes in damp situations, it appears that excessive damps are inimical to the fungus, for its growth is more rapid in proportion as the situation is less damp, until arrived at that certain degree of moisture which is suited both to its production and vegetation. When further extended to dry situations, its effects are considerably more destructive to the timber on which it subsists: here it is very fibrous, and in part covered with a light brown membrane, perfectly soft and smooth. It is often of much greater magnitude, projecting from the timber in a white spongeous excrescence, on the surfaces of which a profuse humidity is frequently observed: at other times, it consists only of a fibrous and thin-coated web irregularly on the surface of the wood. Excrescences of a fungiform appearance are often protruded amidst those already described, and are evidences of a very corrupt matter peculiar to the spots whence they spring. According to the situation and matter in which they are produced, they are dry and tough, or wet, soft, and fleshy, sometimes arising in several fungiforms, each above the other, without any distinction of stem; and when the matter is differently corrupted, it not unfrequently generates the small acrid mushroom. Mr. McWilliam observes, “The fungi arising from oak timbers are generally in clusters of from three to ten or twelve; while those from fir timber are mostly in single plants: and these will continue to succeed each other until the wood is quite exhausted.” Damp is not only a cause of decay, but is essential to it; while, on the other hand, absolute wet, especially at a low temperature, prevents it. In ships this has been particularly remarked, for that part of the hold of a ship which is constantly washed by the bilge-water is never affected with dry rot. Neither is that side of the planking of a ship’s bottom which is next the water found in a state of decay, even when the inside is quite rotten, unless the rot has penetrated quite through the inside. It matters little whether wet is applied to timber before or after the erection of a building. Timber cannot resist the effect of what must arise in either case; viz. heat and moisture, producing putrid fermentation; for instance, in basement stories with damp under them, dry timber is but little better than wet, for if it is dry it will soon be wet; decay will only be delayed so long as the timbers are absorbing sufficient moisture, therefore every situation that admits moisture is the destruction of timber.