Page:Cyclopaedia, Chambers - Supplement, Volume 2.djvu/649

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more fim'ilar to the ufual cuftoms of the workmen. For ' when a carpenter has occafion for a piece of four or five inches fquare, he does not for this purpofe cut a tree that might have furnifhed a piece of a foot fquare, becaufe of the ex- pence i and it too often is their cuftom to ufe fuch pieces as have a very large mare of the blea on them. Not unfrequent- ly indeed they ufe alfo rafters, fawed out of large pieces of Timber; hut thefe are, of all others, the very worft ; and the ufe of them in building ought to be prohibited, the fmall wood being infinitely more advantageous for thefe purpofes. As the degree of drynefs of wood alters its ftrength very much, and as it is alfo very difficult to be affured with any degree of certainty, of the degree of drying or feafoning of Timber ; fmce two trees cut down at the fame time fhall often be found to dry very differently, it appeared to this gentleman much more eligible to take wood perfectly green for his trials, than that in any degree dried or feafoned.

The trees were felled one by one, and were carried into a proper place ; the day after they were felled the carpenters were employed to fquare them, and the joiners to take their exaft dimenfiom; and on the day afterwards, the pieces were put to the proof; and the following is the method in which moft of the experiments were made. Two ftrong treffels were taken, of feven inches fquare, three foot high and three foot long, and ftrengthened in their middle by a^very ftrong piece of Timber. On thefe two treffels were placed the two extremities of the piece of wood which was to be broken ; there were alfo ready feveral fquare iron buckles, or rings, made of round iron work, the largeft of which was nine inches wide, and was made of iron of feven inches about; the next was feven inches wide, and the iron it was made of was five inches round, and others of different degrees below thefe.

The piece of wood deftined for the experiment, was paffed through thefe rings; the larger of which ferved for the thicker pieces, the fmaller for the thinner. Every one of thefe rings had on its upper part and on the infide, a rib nicely filed, of two or three twelfths of an inch broad.

The intent of this was to prevent the ring from inclining one way or other, when in ufe ; and on its under part it had two hooks of iron, of the fame thicknefs with the ring itfelf ; thefe were placed at fuch a diftance from each other, as to form another ring of about nine inches in diameter, into which there was to be put a key of wood of the fame dia- meter, and of four foot long. This key carried a ftrong table of fourteen feet long, and fix feet broad, which was made of rafters of five inches thick, nicely fitted one to the other, and fecured by ftrong bars of wood. This was fuf- pended to the ring by means of the large wooden key, and ferved to place the weights on ; thefe confiffed of three hun- dred quarters of ftones, which were all numbered, and wei»h- ed from twenty-five to two hundred pounds each. Thefe ftones were put upon the table, and a heap of ftones were placed together as long and wide as the table, and as high as was ncceflUry for the breaking of the piece. There was always great care ufed to place the piece and the treffels on a level ; and they were then fattened, to prevent their difplacing themfelves. Eight men were employed con- tinually to fill the table, and began by placing in the center weights, firft of two hundred, then of an hundred and fifty, next of a hundred ; and fo on to twenty-five pounds. Two men placed on a fcaffbld, fufpended in the air by cords placed the fifty and twenty-five pound weights, which could not have been otherwife done, without danger of the mens being crufhed to death ; four other men held firm the four corners of the table, that they might not ftir ; and another, with a long ruler of wood marked how much the piece bent and gave way, as they loaded it with weights ; while another marked the time, and wrote down the weight, which was often, twenty, twenty -five, or twenty -feven thoufand pounds.

Mr. Buffon caufed more than an hundred pieces of wood to be broken in this manner ; fome regular fquare beams, others rafters fawn out of large trunks, befide three hundred bars ; and this great number was but barely fufficient to give a re- gular fcale of the force and ftrength of Timber, in proportion to its feveral lengths and thicknelles.

This gentleman obferved, that a large piece of wood never broke, without firft giving fome notice of it ; that green wood broke with much more difficulty than fuch as was dry ; and l"n general, fuch fort of wood as had a fpring, refifted more than fuch as had none. The blea of the wood, the wood of the branches, that of the top of the tree, and in general all the young wood, is weaker than that which is older. The ftrength of wood is not at all proportioned to its fize ; and a piece which is double or quadruple the thicknefs of an- other of the fame length, is much more than of twice or four times its ftrength ; as for example, there required not quite four thoufand weight to break a piece of Timber of ten foot long, and four inches fquare; and yet there required ten thoufand to break a piece of double that meafure ; and to break one of four times the meafure, it required twenty-fix thoufand weight ; that is, to break a piece of ten foot long, and eight inches fquare. Suppl. Vol. II.

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The fame obfefvation is alfo made good, in regard to the dif- ferent lengths, as Well as the thickneffes. The laws of me- chanics would inform us, that a piece of Timber of eight foot long, ought to bear juft twice the weight of one of fixteen feet. But experiment fhews us here alfo, that the piece of eight foot long carries much more than the double of the other.

The wood which grows quickeft, fuppofing the land the fame, is always the ftrongeft : That which has grown more (lowly, and the annual circles of which, that fej its feveral woody cones, are thin and flender, is always weaker than the wood in which they are thick.

The ftrength of wood is proportionable to its weight, and a piece of Wood of the fame length and thicknefs with another, but confiderably heavier than another, is found ftronger alfo, and that very nearly in the fame proportion. This is a re- mark of no fmall confequence ; and, as the general weight of wood is eafily known, it may fave the expence of too much wood in building, as well as the danger from the greater error of ufing too little.

This accurate obferver takes notice, that fome may imagine his experiments to be the lefs conclufive, that the Timber he broke was loofe at the ends ; and that in a wall, or building, it is ufually fixed at both ends. But he obferves, that the difference of this circumftance is not worth confidenng, for that the piece fixed in a wall is liable to bend in the middle, before it breaks ; and that the fixures have no power to re- tain the ends, when they are pulled by this force. 1 his gentleman took no notice of any pieces which had faults, as many of thofe he tried had, and broke with much lefs than their proportion of weight ; but confined his obfervations and reaibnings only to fuch pieces as were perfectly found and firm.

It might be fuppofed, that fo great a number of trialsas thefe, and that on fo eafy a fubject, ftiould have left nothing farther to be done in it ; but the author himfelf willingly acknow- ledges that there is yet much to be known. The proportion of the longitudinal ltrength of wood to its tranfverfe, is a fub- je£t deferving enquiry ; as wood in buildings is often ufed in very different portions, and where the force a&s upon it, not tranfverfely, but longitudinally or obliquely in different direc- tions. Mem. Acad. S'cienc. Par. 1740. Seafoning c/Timber, a term ufed by our hufbandmen, to ex- prefs the preparing Timber after it is felled, for cutting and working up for uk.

As foon as felled, it fhould be laid up in fome dry airy place, but out of the reach of too much wind or fun, which, when in excefs, will fubject it to crack and fly. It is not to be fet upright but laid along, one tree upon another, only with fome Ihort blocks between, to give it the better airing, and pre- vent its becoming mouldy, which will rot the furface, and produce mumrooms on it. Some perfons daub the trees all over with cow-dung, which occafions their drying equally, and prevents their cracking, as they are otherwife very apt to do.

Some recommend the burying Timber in the earth, as the beft of all ways of feafoning it ; and others have found it a fine prefervative to bury their Timber under the wheat in their granaries ; but this cannot be made a general practice. In Norway, they feafon their deal planks, by laying them in falt-water for three or four days, when new fawed, and then drying them in the fun ; this is found a great advantage to them ; but neither this, nor any thing elfe, can prevent their fhrinking.

The feafoning Timber by fire Is the beft way of all, for piles and other pieces that are to ftand under the earth, or water. The Venetians firft found out this method, and the way they do it in is this ; they put the piece to be feafoned into a ftrong and violent flame, in this they continually turn it round by means of an engine, and take it out when it is every where covered with a black coaly cruft ; by this means the internal part of the wood is fo hardened, that neither earth nor water can damage it of a long time afterwards. This method is praitifed in many places for feafoning the pofts for paling of parks, &c, and has this to recommend it, that in the very oldeft ruins we have ever been acquainted with, there have been found many times pieces of charcoal, all of which has been found uninjured, though buried in the earth for ever fo many ages. Mortimer's Hufbandry, V. 2. p. 132.

When Timber has been properly felled, and feafoned, by be- ing expofed to a certain degree of air and fun, there is yet a method of preferving it, much better than it could other- wife be, by external applications. When they are wrought and fixed in their places, the rubbing them over with tar or linfeed-oil, has been found of great ufe. The antients, par- ticularly Virgil and Hefiod, advife the fmoak-drying of all in- ftruments made of wood, by hanging them up in the chimnies where wood fires are ufed. The whole benefit arifing from this, feems to be the oil of the wood that is burnt, entering into the pores of the other as carried up in the fmoak. But it would be a much more cleanly and eafy method to apply fome oil of this kind at once, and in any quantity that fhould be judged requifite. Mortimer's Hufbandry, V, 2. p. 104.

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