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cation of great value, fcems to threaten a general lofs of them ; but as trees, thus naturally crooked and bent, are of value, it is a laudable attempt to try at the finding a regular method of producing fuch ; and this is eafily practicable, by following the fame methods by which thefe wild ones become (o. They wholly owe their figure to the cattle's biting off their tops while young, and afterwards biting off again the tops of the flioots from the firft wound. In this manner, if a number of young trees, fet apart for the experiment, have their tops cut off at two, four, fix, eight, ten, and twelve feet from the ground, and four years afterwards the fhoots from thefe itunted tops are again cut in the fame manner, the trees will be found afterwards to grow up in all the irregularly crooked figures that can be conceived, and by this means a fupply of naturally crooked wood may be railed for all the occalions of fliip-building, with infinitely greater eafe and more certainty than by the method propofed by fome, of bending them down with weights tied to their tops while young. As to the fupply of young wood in the place of what is cut down, there are fome circumftances which have not had the attention paid to them which they deferve. The fpring frofts, which come on at a time when the fhoots, by which nature is to raifc the fupply for what is cut down, are juft preparing to grow, are of prodigious injury, and do not lefs mifchief to thefe than to the young fhoots of garden plants, though the diftant hope of the fucceffion to the proprietor, and ufualiy alfo the diftance of place, and want of repeated obferva- tion, occalions its not being perceived. This, however, may, in a great meafure, be guarded againft. Frequent expe- riments, and repeated obfervations, prove, that the mifchief done by thefe frofts, affect in a much greater degree thofe flioots which are expofed to the fouth, than thofe which face the north ; and that it is greatly more powerful againft fuch as are wholly expofed to the wind, than againft fuch as are fheltered. Thefe known circumftances may give the hint to a method of laving, at leaft a great part of a wood to be fel- led, from this deftruction, to its renewal, by the making it a rule to begin cutting down on the north fide ; and, as the whole felling is a work of fome years, the ftanding wood of every feafon will defend the young fhoots of the newly cut flumps the following fpring, not only from the fouth expo- sure, but will Inciter them alfo from the wind. Many prudent managers have made fine eftates of their cop- pice-woods, by regularly felling a certain portion every year, and providing for a renewal of the firft cutting, againft the felling of the laft portion, by proportioning the time of growth to the quantity to be cut every year ; and there is great intcrcft to be made of a true knowledge of the growth of wood in this manner. Whoever obferves the growth of young trees, will find that the fecond year's growth is much more confiderable than that of the firft ; the third year is more than that of the fecond, and fo on for many years; the yearly growths of young wood greatly encreafing every feafon up to a certain time or age of the tree, after which the increafe in bulk, by growth, becomes gradually lefs. The great advan- tage to be made of coppice-wood, would be by knowing this interefting period, and feizing on it, always to cut down the trees juft at that time when they arrived at the end of their quick growth, and fo fetting nature to work with new lhoots, to employ the fame fpeed on enriching again the owner. Regular obfervation and experiment alone can afcer- tain this happy period ; but any man who has much coppice- wood upon his eftate, may aflure himfclf of it, by cutting a given quantity every year, for ten years fucceffively, and then carefully reviewing the differences of the yearly produce. Memoirs Acad. Scienc. Ann. 1739. Strength ef 'Timber. The celebrated Monfieur de Buffon has attempted a rational calculation of the ftrength of Timber, ufed in building, on the principles of his excellent demonftra- tion of the growth of trees. See the article Wood. He has proved that all wood confifts of a feries of woody cones laid in woody beds, all along the tree, and connected by a fort of reticular work of much lefs ftrength than their own fubftance. He infers hence, very juftly, that when there are, in any fpecies of wood, more of thefe cones, which fhew themfelves in fo many concentric circles in a tranfverfe fection of the wood, and confequently more of thefe weaker reticular fubftauces which connect them, that wood muft ne- ceflarily be weaker than fuch in which, in an equal thick- nefs, there are much fewer of thefe cones, and confequently fewer alfo of thefe weaker inter-fpaces.
If the irregularities of the ftru&ure of wood were only thefe, it would be eafy to compute, from the ftrength of a ftaff of an inch diameter, of any wood, what would be that of a beam of the fame, of any given fize ; but this is by no means the cafe ; for the difpofition of the cones, beds, and interfticial matter, is by no means the fame Ih a large beam, formed by the workman, as in a little branch in its natural ftate of growth.
This is eafily conceived, when we confider that, in order to make a beam of Timber* they fquare a branch or part of the tree, or cut off four cytindric pieces from it, of what is called the blca. The heart of the wood lies now in the middle of
(He beam ; this is the firft woody bed, which all the others' fucceffively cover, in form of lb many concentric circles ; The larger of thefe concentric circles is of the diameter of the whole piece of Timber now cut, and the circles, which na- turally furrounded this, are now no longer perfect, but form only portions of circles, diminifliing all the way to the ribs of the beam. A beam, thus fquared, is therefore compofed of a cylindric compleat piece of firm and hard wood, and of four angulaf portions, cut out of a much lefs folid and firm wood. This is the condition of a beam, fquared out of a large arm of a tree, and made nearly of the diameter of that arm ; but a beam of like fize, cutout of the trunk of a larger tree, or out of one fide of luch a trunk, is a very different thing, and is compofed only of the longitudinal fegments of the annual woody beds ; and thefe fegments fometimes He parallel to one of the furfaces of the beam, and fometimes more or lefs in- clined to one or the other fide ; they are alfo fometimes longer, fometimes fhorter ; and fometimes more, fometimes lefs, cut into the woody beds of the trunk of the tree they were cut from, and the beam is confequently more or lei's ftrong accordingly.
There are alfo two pofitions in a beam, the one of which is much ftronger than the other ; for thefe fegments of the woody beds of the tree, form fo many parallel flats. If you place the beam in fuch a pofition that thefe planes or fiats are vertical, it will refill: a much greater force than if they are placed horizontally in the pofition of the beam. From thefe obfervations, it is eafy to fee how very fallacious the calculations and tables of the ftrength of different kinds of limber muft be, while the authors have formed all their (y- ftems upon experiments made on pieces of one or two inches in diameter, and have given themfelves no trouble to enquire into either the number or difpofition of the woody beds in thofe pieces, nor the directions in which the feveral beds were found on breaking them. Thefe, however, are extremely neceffary and eflential circumftances ; nor are thefe the only variations which they have omitted to obferve ; there are per- haps others equally neglected in all fuch attempts, and yet equally neceffary to make them of any real ufe. Young wood has always much lefs ftrength than the older, and there is a great difference between the pieces cut out of the foundeft part of the body of a tree, and thofe from the young branches of the fame tree. A piece cut out of the outer part of the trunk of a tree, near the blea, is alfo much weaker than one of equal diameter, cut out of the center or heart of a tree. The dif- ferent age, and degree of drynefs in wood, makes alfo great differences in its ftrength ; for a green branch of a tree is al- ways more tough, and lefs eafily broken, than a dry one : And the time employed in putting wood to the trial, as to its ftrength, is alfo a circumftance very neceffary to be con- fidered ; fince a beam of Timber, which will fuftain a certain weight for a few minutes, will not fuftain it perhaps for as many hours ; and experiment has proved, that wood will of- ten bear, for a fhort time, a weight it cannot bear for a longer time ; a beam having been found to bear a certain weight for one day, and the fame beam afterwards having been broken in fix months time by only two thirds of that weight. It is eafily feen, from thefe confederations, that all the calculations in which thefe confiderations have been ne- glected, can be of very little certainty ; and that to make fuch as may be certainly depended on, is a work of great diffi- culty.
As the advantages refulting from fuch calculations are, how- ever, yet greater than the difficulties attending the attempt, this gentleman was encouraged to purfue the enquiry to the utmoft. He caufed certain pieces of wood to be broken by a determinate force, and calculating, according to that, what ought to be the force to break fuch as were much larger ; he tried afterwards what was the real force that would break them, and how far his calculations approached to the truth ; in this he ever found extremely great differences between calculation and reality ; and after many repeated trials, was never able to bring the fact: any thing near the calculations. And trying, according to the fame rules, others of greater lengths and fchickneffes, in proportion to the ftandard piece, he found the calculations as far faulty as be- fore. The difappointment in thefe attempts determined him, however, to try a compleat courfe of experiments, from which a certain table of the force required to break the dif- ferent kinds of wood might be eftablifhed, to which both him- felf, and the reft of the world, might refer to upon occafion. And this he executed in the following manner. He began by choofing in a certain fpot of one of his own woods an hundred oaks, which were all found and lively growing trees, and ftood as near to one another as they conveniently could. This caution was ufed, that the wood might be all of the growth of the fame foil, fince it is well known that the growths of different foils are in the fame fpecies of wood, of very different ftrengths, and miftakes of this kind might have rendered the whole attempt fruitlefs. Thefe oaks were all of the fame fpecies, and were in fize from two foot and a half to five foot round. They were chofen purpofely of different growths, or fizes, that the experiments might be the 3 more
