By comparing Experiments 1 and 3, the strength appears proportional to the breadth. Experiments 3 and 4, show the strength proportional to the square of the depth. Experiments 1 and 5, show the strength nearly in tie inverse proportion of the lengths, but with a sensible defi ciency in the longer pieces. Experiments 5 and 7; show the strengths proportional to the breadths and the square of the depths. Experiments 1 and 7, show the same thing, compounded with the inverse proportion of the length ; the deficiency relative to the length is not so remark able here. Experiments 1 and 2, and Ex periments 5 and 6, shOw the increase of strength, by fastening the ends, to be in the proportion of 2 to 3. The theory gives the proportion of 2 to 4. But a dif ference in the manner of fixing may pro duce this deviation from the theory, which only supposed them to be held down at places beyond the props, as when a joist is held in the walls, and also rests on two pillars between the walls. We shall here give an abstract of M. Buf fon's experiments. He relates a great number which he had prosecuted during two years on small battens. He found that the odds of a single layer, or part of a layer, more or less, or even a different disposition of them, had such influence, that he was obliged to abandon this me thod, and to have recourse to the largest beams that he was able to break. The fol lowing table exhibits one series of expe kiments on bars of sound oak, clear of knots, and four inches square. This is a specimen of all the rest. Column 1, is the length of the bar in feet clear between the supports. Column 2, is the weight of the bar (the second day after it was fell ed) in pounds. Two bars were tried of each length. Each of the first three pairs consisted of two cuts of the same tree. The one next the root was always found the heaviest, stiffest, and strongest. In deed, M. Buffon says, that this was inva riably true, that the heaviest was always the strongest ; and he recommends it as a sure rule for the choice of timber. He finds that this is always the case when the timber has grown vigorously, forming very thick annual layers. But he also ob serves, that this is only during the ad vances of the tree to maturity ; for the strength of the different circles approach es gradually to equality during the tree's healthy growth, and then it decays in these parts in a contrary order. Our tool makers assert the same thing with re spect to beech ; yet a contrary opinion is very prevalent ; and wood with a fine, that is, a small grain, is frequently prefer red. Perhaps no person has ever made the trial with such minuteness as M. Buf. on, and much deference is thought to be due to his opinion. Column 3, is the num ber of pounds necessary for breaking the tree in the course of a few minutes. Co lumn 4, is the inches which it bent down before breaking. Column 5, is the time at which it broke, Mr. George Smart, well known for his practical knowledge of mechanics, in al most every department, says, that after making many experiments on timber, and comparing them with those of Belidore, Bufibn, &c. the differences were so great, that it would be wasting time to enume rate them. He therefore mentions some useful observations necessary to he known by all those mechanics who use timber ; and points out some evident errors in a table of Belidore's, supposed to be the result of the best set of experiments ever produced in transverse strains. He tells us, that a bar of wood, thirty-six inches long, and one inch square, supported at the ends by two props, will break with a weight of 187 pounds on the middle, Wit is loose at the ends if the ends are firm ly fixed, it will require 283 pounds to break it. "This appeared to me," says Mr. Smart, "so great an error, that I was induced to put little or no confidence in many of his experiments; and, in consequence, I made two laths of fir, of the same dimensions, one with a strong shoulder at each end, to prevent it bending, which having firmly fixed in a frame, it carried a weight more than ten times greater than that which was loose." The fibres of timber requiring so great a force to tear them asunder in a vertical direction, and being easily broken by a transverse strain, when compared to that of a rope carrying nearly an equal weight in all directions, opens a wide field for useful experiments. All timber trees have their annual circles, or growths, which vary greatly according to the soil and exposure to the sun. The north-east side of th e trees (being much smaller in the grain than the other parts which are more exposed to the sun) is strongest,for any column that has a weight to support in a vertical direction ; because its hard cir cles, or tubes, are nearer each other, and the area contains a greater quantity of them ; nor are they so liable to be com pressed by the weight, or to slide past each other, as when they are at a greater distance. On the other hand, this part of
the tree is not fit for a transverse strain ; because the nearer the hard circles are to each other, the easier the beam will break, there being so little space between them, that one forms a fulcrum to break the other upon ; but that part of a tree, the tubes of which are at a greater dis tance, or of a larger grain, is more elas tic, and requires a greater force.to break it; because the outside fibre on the vex side cannot snap till the next one is pressed upon it, which forms the fulcrum to break it on. It is generally observed in large timbers, such as masts, that the fracture is seldom on the convex, but usually on the concave side ; which is owing to the fibres on the concave side being more readily forced past each other, and those on the convex being so difficult to be torn asunder, that they cannot snap, in consequence of the large ness of the segment of the circle they describe when on the strain. The curve described by the inner layers of the wood being so large, and indeed little less than a straight line, cannot form a fulcrum to break the outer ones upon ; and as the convex side, or that on which the fibres are extended, ought to be al ways free from any amortise or incision on the outside, the strength decreases as it approaches the centre. Mr. Smart has, in a paper in the " Repertory," given di rections how to cut and join timber so as to have the greatest strength, and to turn to the greatest advantage, of having the best part of the tree in the place where the hardness and strength are most want ed, viz. in the corners which form the abutments ; whereas the same tree, squared into a parallel beam, would have been much smaller, and the soft or sappy parts of the wood exposed to the action of the air and moisture. In flush framing it is observable, that the failure of all tim ber in old buildings has commenced much sooner than they otherwise would have done, owing to the sappy wood be ing at the corners of the principal beams, which soon decays, as its spongy quality attracts the moisture ; whereas the heart, especially of oak, will be as sound as the first day it was used.
As all beams take their weight horizon tally, or on any transverse bearing; hai:e their principal strain on the upper and lower surface, every workman ought to guard against having sap in beams, be cause, if they do not immediately decay, they shrink, so as to let loose all the framing, and soon cripple the building or machine ; but on Mr. Smart's plan, the sappy part of the wood is excluded from what would cause its decay, and the tim ber increased in quantity is considerably more than the extra labour and expense.
Tim salt trees, in law, are properly oak, ash, and elm. In some particular coun tries, by local custom, other trees being commonly there made use of for building, are considered as timber. . Of these, be ing part of the freehold, larceny cannot be committed ; but, it' they be severed at one time, and carried away at another, then the stealing of them is larceny. And by several late statutes, the stealing of them in the first instance is made felony, or incurs a pecuniary forfeiture. For the better preservation of roots, shrubs, and plants, it is enacted, by 6 George Ill. c. 48, that every person convicted of damag ing, destroying, or carrying away any timber-tree, or trees, or trees likely to become timber, without consent of the owner, &c. shall forfeit for the first of fence not exceeding 201. with the charges attending; and on non-payment shall be committed for not more than twelve, nor less than six months ; for the second offence, a sum not exceeeing SOL and on non-payment shall be committed for not more than eighteen, and not less than twelve months; and for the third offence, is to be transported for seven years. AU oak, beech, Chesnut, walnut, ash, elm, cedar, fir, asp, lime, sycamore, and birch trees, shall be deemed and taken to be timber trees, within the time meaning and provision of this act. Persons con victed of plucking up, spoiling, or taking away any root, shrub, or plant, out of private cultivated ground, shall forfeit, for the first offence, any sum not exceeding 40s. with the charges ; for the second of fence, a sum not exceeding 51. with the charges ; and for the third offence, are to be transported for seven years. A power is given to justices of the peace to put this act in execution.