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Dry Rot

timber, timbers, tree, heart, decay, air, juices, preparing, beams and sound

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DRY ROT. A term applied to that rapid decay of timber by which its substance is converted into a dry powder, which issues from minute tubular cavities resembling the borings of worms. On the causes of this decay numerous volumes have been written, and nostrums for its prevention or " cure" have been proposed without number, yet the dry rot continues to ravage our houses, and to destroy our ships. It has been said that moist and warm situations, where the circulation of the air is impeded, is the generating cause of this disease, and that the effluvia from timber so diseased will carry its effects to the circumjacent timber ; and that any sort of wood, dry as well as damp, so exposed, will be soon destroyed. Timber once infected cannot be restored, and the only remedy lies in cutting away the diseased parts, to prevent the extension of the evil to the remainder ; and to effect even the latter a free circulation of air must be admitted, and the parts be washed over with a strong solution of iron, copper, or zinc. Patents have been granted for various applications of the latter, as preventives of the dry rot, the distinguishing features of the processes therein employed consisting in first preparing the timber by a good steaming, or drying out of the sap, and afterwards injecting, soaking, or boiling the timber in a solution of copperas, or other metallic salt. The following observations on this important subject were some time since addressed to the editor by Mr. John Gregory, who an experienced and observant shipwright; and as they appear to mark out clearly the true cause of, and to suggest a very simple remedy for, the evil, it is right to give them a place in this work. Mr. Gregory says, "Instead of squaring a piece of timber according to the usual method, by leaving the heart of the tree in the centre, my plan is to saw it right down the middle, through the heart, into two equal parts, immediately after the tree is felled ; and my reasons for this I will now endeavour to explain to the best of my ability. It is, I believe, a well-known fact, that a tree does not, literally speaking, die on receivingjthe final stroke of the axe, but that it continues for a long period afterwards to vegetate, though less vigorously. At length, however, the sap ceases to circulate, the pores become closed, and the juices of the tree thus shut up undergo decomposition, and lay the foundation of dry rot. It is well known that a man who dies in a full habit of body soon decays ; the same effect takes place in a tree full of sap, unless we adopt the same method with respect to it as the Egyptians practised with the human body, viz. that of depriving it of all moisture, which process would give to our timber a durability almost everlasting. My mind has been long impressed with this idea, which has been confirmed by my having recently noticed that several of the timbers in a very ancient public building, which had been sawn originally in the manner I have proposed, were perfectly sound, although they had withstood the dilapidating hand of time for seven hundred years ; while other timbers in the same building, which had not been so cut, but apparently squared out with the heart in the centre, were perfectly rotten.

That the dry rot is certainly caused by the juices being enclosed in the heart of the timber, I have had frequent opportunities of observing during my long practical experience in the repairing of ships. In the frame of a ship in which such large quantities of timber are employed, I have uniformly noticed, First, that the decay commences in the run fore and aft, which is owing to the timbers being fitted so close together at the heels or lower ends. The evil being thus enclosed in the hearts of the timbers, and the air having no access to the exterior of them to carry off the moisture by evaporation, internal decay is the necessary consequence. I have sometimes witnessed these parts of the frame of a ship in such a rotten state as to have been justly compared by the workmen to a heap of manure. Secondly, those timbers in the midships that have been bored off with the outside planks are not so affected, which I attribute to the circumstance of the holes admitting a current of air through them, the destructive juices being thereby carried off'. Thirdly, it frequently happens that the floor timbers of an old ship are found, on breaking up, to be nearly as sound as they were when first put in. Their preservation seems to be owing to the effect of the salt water which constantly laves over them, causing them to become in a manner pickled; or it may be, that the salt entering into the composition of the wood, the destructive effects of its natural juices are thereby prevented. Fourthly, the planks in the bottom, nearest to the timbers, take the infection first ; and where the tree-nails are not close, the disease rapidly extends endways of the grain. Fifthly, those parts of the deck planks that lie upon the beams are those which are first infected with the rot, the cause of which is evident, as those parts that are between the beams are generally quite sound. Sixthly, in the beams of ships the decay usually commences in the internal parts, which is decidedly owing, in my opinion, to the erroneous method of preparing the timber, as before mentioned ; but when timber, so prepared, is used, I would recommend, as the best preventive of the rot, that a few holes be bored through the beam fore and aft, and, what would still add to the benefit, to bore another hole lengthways of the grain, to meet those which are bored crossways. But the best preventive, I am confident, would be the adoption of my mode of preparing the timber, namely, to saw it lengthways right through the heart, by which not only much greater durability would be obtained, but great economy in the consumption of the timber, as well as a great increase of strength, which I will proceed to ex plain by reference to the annexed figure, which exhibits an end view or section of a piece of timber. Having procured a log of the shape required, first cut off the two slabs b b, and then divide the remainder a a into two equal parts.

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