Since writing the above, we have looked over some experiments by Messrs Barlow, Beaufoy, Couch, and others, on the strength of timber. These show so much discrepancy of result, as leads us to conclude, {222} that experimenters have not attended sufficiently to the structure and nature of the timber, the position and quality of the different layers, &c. Take, for example, the stem of a tall tree, 100 years old: At the cross section, it is found to consist of a certain number of layers of matured timber, and of sap timber. These layers having been gradually formed, the external, after those more internal have partly dried, and the internal and matured wood being also filled up to more solid consistency than the external, the stem, on being barked, contracts in drying much more externally than internally. As soon as the surface has dried, the outer layers contracting laterally are not sufficient to surround the undried internal layers, thence split in longitudinal rifts; and as the drying proceeds inwards, the cracks deepen till they reach nearly to the heart—these rifts, when the timber is thoroughly dry, being generally wider in the sap timber than in the matured, more than in the proportion of the size of the respective circles. This effect of drying is what every body is acquainted with.
Besides lateral contraction, there is also a disposition to contract longitudinally by drying, much greater in the external than internal layers. While the tree is undivided, this greater contraction of the {223} exterior layers is prevented, by the adhesion to the drier more filled up central column (which probably had contracted a little during the formation of the exterior sap-wood layers), the contractile force of the exterior balancing equally around this central column. Should this balance be destroyed by the stem being cleft up the middle, the longitudinal contraction will immediately take place, and the two halves will bend outward, from the outside layers contracting more than the inside layers. We have seen an ash tree rend up the middle from the cross section above the bulb, nearly to the top, on being cut across in felling, owing to the longitudinal contractile force of the exterior existing even before drying.
Should the dried stem of a tree, of considerable length, be laid hollow, supported at each end, the outside layer being stretched almost to breaking by the longitudinal contraction being greatest in the outermost part, a very small weight, aided by a slight jerk or concussion, may be sufficient to burst the outside layer on the lower side, the outside layers on the upper side not standing out as a support above, but combining their contractile force with gravity to rend the lower. As the outer layer gives way, the strain is thrown concentrated upon the next outermost, which also gives way, and the beam is broken {224} across in detail. In like manner, when the direct longitudinal strength is tested, the external circles being in greater tension than the internal, the tightest parts of the log will give way in succession, like a rope with strands of different degrees of tightness; yet the lateral adhesion of the layers will have considerable effect in strengthening the mass.
The above explains the fallacy of estimating the longitudinal strength of a thick piece of timber from experiments with small shreds; it likewise explains how a large unbuilt mast is so easily sprung; wherefore a beam round as grown will be rendered stronger as a beam by being formed into a hollow cylinder, by boring out the central part; and also how a square log will be strengthened as a beam, by cleaving it up the middle, and placing the two pieces on edge, with their outside or backs together. In the latter case, the middle, by being turned outside, and exposed to the air, will contract more than what it would do shut up and covered by the exterior wood, especially if resinous pine timber, which continues to contract for many years, owing to the resin, when exposed to the air, gradually drying or undergoing some change, by which it is diminished in size, and rendered similar to amber. {225}
Consideration of the difference of tension of the concentric layers, from the difference of disposition to contract by drying, modified by the difference of position in which these layers may stand, when supporting weights and bearing strain, with the various qualities of timber of the same kind of tree, from variety, age, soil, climate, or from being taken from near the outside or heart, or butt or top, will, we think, account for the contrariety of results which unphilosophical experiments have afforded.
VI.—STEWARDS PLANTER’S GUIDE, AND SIR WALTER SCOTT’S CRITIQUE.
We have noticed that a sensation has been produced in a certain quarter, particularly among persons of a certain age, by a publication of Sir Henry Steuart of Allanton, on removing large trees, eked out by a very clever article in the London Quarterly, on Landscape Gardening, ascribed to Sir Walter Scott.
It may seem unnecessary to direct the attention of the public again either to this volume or its subject, both of which have already engaged the public attention to a degree greatly beyond their value and importance; but Sir Henry, with all his foppery and parade of decorating parks, approaches, and lawns, and all that sort of chateau millinery, has now and then risen above his subject, and not only given us several hints useful in rural economy, but has also pretensions to have brought out some facts hitherto but imperfectly known, and to have traced them to general principles. {227}
It is curious to remark of how much greater importance the elder part of society—those upon whom wealth has at length devolved, are generally held. Any device, however trifling, which can in any way divert the fancy, pamper the lingering senses, or patch up the body of our second childhood, is infinitely more useful to the discoverer, and meets with higher patronage and more eclat, than what is of a thousand times more consequence to the young. Now, if this were the fruit of filial love, all would be very well—we would idolize the picture: but when we see these discoveries only patronized by the old themselves, in the merest egotism, we blush for our patriarchs, and wonder if time and suffering will be spent as unprofitably upon ourselves.
We wonder much what fascination can exist to a mind of so much ability and culture as that of Sir Henry Steuart, in decorating a few dull unprofitable acres,—causing a few bushes and bush-like trees to change place from one side of a dull green to the other!—laying digested plans of action, embracing a great number of years, to accomplish this very important feat, which most probably the next heir will make the business of his life to undo, by turning them back to their old quarters, if he does not, with more wisdom, grub them out {228} altogether as cumberers of the soil! For ourselves, we would rather baa with the silly sheep, and nibble the turf, than pass our time in acting over this most pitiful trifling, or in publishing a memorial of our shame. We know not how others are affected, but there is no other place on earth where we have felt such oppression and weariness, as in the extensive smoothed park and lawns around the country seat. We sicken under the uniformity of the heavy-looking round-headed trees,—the dulness of the flat fat pasture, undecorated by a single weed,—the quiet stupid physiognomy of the cattle,—the officiousness of the sleek orderly menial. It may be we are very destitute of taste in this; here every thing is experiencing satiety of sensual enjoyment, is full to repletion; every thing has been sedulously arranged to please, and we ought certainly to admire; but we have no sympathy with such a scene.