424 



STRENGTH OF MATERIALS. 



rule for giving strength will be found to be, to 

 cause the force tending to destroy, to act, as equally 

 as may be, on the whole resisting mass, at the same 

 time, and with as little mechanical advantage as 

 possible. In longitudinal compression, as produced 

 by a body on the top of a pillar, the weight, while 

 the support remains straight, can only destroy the 

 support by crushing it in opposition to the repul- 

 sion and impenetrability of all its atoms. Hence a 

 very small pillar, if kept perfectly straight, sup- 

 ports a very great weight ; but a pillar originally 

 crooked, or beginning to bend, resists with only 

 part of its strength ; for the whole weight above is 

 supported on the atoms of the concave side only, 

 which are therefore in greater danger of being over- 

 pressed and crushed, while those on the convex 

 side, separated from their natural helpmates, are in 

 the opposite danger of being torn asunder. The 

 atoms near the centre, in such a case, are almost 

 neutral, and might be absent without the strength 

 of the pillar being much lessened. Long pillars or 

 supports are weaker than short ones, because they 

 are more easily bent ; and they are more easily bent 

 because a very inconsiderable, and therefore easily 

 affected, yielding between each two of many atoms, 

 makes a considerable bend in the whole ; while in 

 a very short pillar, there can be no bending with- 

 out a great change in the relation of proximate 

 atoms, and such as can be effected only by great 

 force. The weight or force bending any pillar 

 may be considered as acting at the end of a long 

 lever, reaching from the end of the pillar to its cen- 

 tre, against the strength resisting at a short lever 

 from the side to the centre. The strength, there- 

 fore, has relation to the difference between these. 

 Shortness, then, or any stay or projection at the 

 side of the pillar, which, by making the resisting 

 lever longer, opposes bending, really increases the 

 strength of a pillar. A column with ridges project- 

 ing from it is, on this account, stronger than one 

 that is perfectly smooth. A hollow tube of metal 

 is stronger than the same quantity of metal in a 

 solid rod, because its substance, standing farther 

 from the centre, resists with a longer lever. Hence 

 pillars of cast-iron are generally made hollow, that 

 they may have strength with as little metal as pos- 

 sible. In the most perfect weighing-beams for 

 delicate purposes, that there may be the least pos- 

 sible weight with the required strength, the arms, 

 instead of being of solid metal, are hollow cones, in 

 which the metal is not much thicker than writing 

 paper. Masts and yards for ships have been made 

 hollow, in accordance with the same principle. In 

 nature's works, we have to admire numerous illus- 

 trations of the same class. The stems of many 

 vegetables, instead of being round externally, are 

 ribbed or angular and fluted, that they may have 

 strength to resist bending. They are hollow, also, 

 as in cornstalks, the elder, the bamboo of tropical 

 climates, &c., thereby combining lightness with 

 their strength. A person who visits the countries 

 where the bamboo grows, cannot but admire the 

 almost endless uses to which its straightness, light- 

 ness and hollowness, make it applicable among the 

 inhabitants. Being found of all sizes, it has merely 

 to be cut into pieces of the lengths required for any 

 purpose; and nature has already been the turner, 

 and the polisher, and the borer, &c. In many of 

 the Eastern islands, bamboo is the chief material of 

 the ordinary dwellings, and of the furniture, the 

 fanciful chairs, couches, beds, &c. Flutes and other 

 wind instruments there are merely pieces of the 



reed, with holes bored at the requisite distances. 

 Conduits for water are pipes of bamboo; bottles 

 and casks for preserving liquids are single joints of 

 larger bamboo, with their partitions remaining; and 

 bamboo, split into threads, is twisted into rope, &c. 

 From the animal kingdom, also, we have illustra- 

 tions of our present subject the hollow stiffness 

 of the quills of birds; the hollow bones of birds; 

 the bones of animals generally, strong and hard, and 

 often angular externally, with light cellular texture 

 within. &c Transverse Pressure. When a hori- 

 zontal beam is supported at its extremities, its 

 weight bends it down more or less in the middle, 

 the particles on the upper side being compressed, 

 while the parts below are distended; and the bend- 

 ing and tendency to break are greater, according as 

 the beam is longer and its thickness or depth is less. 

 The danger of breaking, in a beam so situated, is 

 judged of, by considering the destroying force as 

 acting by the long lever reaching from the end of 

 the beam to the centre, and the resisting force or 

 strength as acting only by the short lever from the 

 side to the centre, while only a little of the sub- 

 stance of the beam on the under side is allowed to 

 resist at all. This last circumstance is so remark- 

 able, that the scratch of a pin on the under side of 

 a plank resting as here supposed, will sometimes 

 suffice to begin the fracture. Because the resisting 

 lever is small in proportion as the beam is thinner, 

 a plank bends and breaks more readily than a beam, 

 and a beam resting on its edge bears a greater 

 weight than if resting on its side. Where a single 

 beam cannot be found deep enough to have the 

 strength required in any particular case as for sup- 

 porting the roof of a house several beams are 

 joined together, and in great variety of ways, as is 

 seen in house-rafters, &c., which, although consist- 

 ing of three or more pieces, may be considered as 

 one very broad beam, with those parts cut out 

 which do not contribute much to the strength 

 The arched form bears transverse pressure so ad- 

 mirably, because, by means of it, the force that 

 would destroy, is made to compress all the atoms 

 or parts at once, and nearly in the same degree. 

 The atoms on the under side of an arch, resting 

 against immovable abutments, must be compressed 

 about as much as those on the upper side, and can- 

 not therefore be torn or overcome separately. The 

 whole substance of the arch, therefore, resists, al- 

 most like that of a straight pillar under a weight, 

 and is nearly as strong. To be able to adapt the 

 curve to the size of an arch, and to the nature of 

 the material, requires in the architect a perfect ac- 

 quaintance with measures, &c. An error which has 

 been frequently committed by bridge-builders is, 

 the neglecting to consider sufficiently the effect of 

 the horizontal thrust of the arch on its piers. Each 

 arch is an engine of oblique force, pushing the pier 

 away from it. In some instances, one arch of a 

 bridge falling, has allowed the adjoining piers to be 

 pushed down towards it, by the thrust, no longer 

 balanced, of the arches beyond, and the whole 

 structure has given way at once, like a child's 

 bridge built of cards. It is not known at what 

 time the arch was invented, but it was in compara- 

 tively modern times. The hint may have been 

 taken from nature; for there are instances, in al- 

 pine countries, of natural arches, where rocks have 

 fallen between rocks, and have there been arrested 

 and suspended, or where burrowing water has at 

 last formed a wide passage under masses of rock, 

 which remain balanced, among themselves, as an 



